egraph-based midend: draw the rest of the owl (productionized). (#4953)

* egraph-based midend: draw the rest of the owl.

* Rename `egg` submodule of cranelift-codegen to `egraph`.

* Apply some feedback from @jsharp during code walkthrough.

* Remove recursion from find_best_node by doing a single pass.

Rather than recursively computing the lowest-cost node for a given
eclass and memoizing the answer at each eclass node, we can do a single
forward pass; because every eclass node refers only to earlier nodes,
this is sufficient. The behavior may slightly differ from the earlier
behavior because we cannot short-circuit costs to zero once a node is
elaborated; but in practice this should not matter.

* Make elaboration non-recursive.

Use an explicit stack instead (with `ElabStackEntry` entries,
alongside a result stack).

* Make elaboration traversal of the domtree non-recursive/stack-safe.

* Work analysis logic in Cranelift-side egraph glue into a general analysis framework in cranelift-egraph.

* Apply static recursion limit to rule application.

* Fix aarch64 wrt dynamic-vector support -- broken rebase.

* Topo-sort cranelift-egraph before cranelift-codegen in publish script, like the comment instructs me to!

* Fix multi-result call testcase.

* Include `cranelift-egraph` in `PUBLISHED_CRATES`.

* Fix atomic_rmw: not really a load.

* Remove now-unnecessary PartialOrd/Ord derivations.

* Address some code-review comments.

* Review feedback.

* Review feedback.

* No overlap in mid-end rules, because we are defining a multi-constructor.

* rustfmt

* Review feedback.

* Review feedback.

* Review feedback.

* Review feedback.

* Remove redundant `mut`.

* Add comment noting what rules can do.

* Review feedback.

* Clarify comment wording.

* Update `has_memory_fence_semantics`.

* Apply @jameysharp's improved loop-level computation.

Co-authored-by: Jamey Sharp <jamey@minilop.net>

* Fix suggestion commit.

* Fix off-by-one in new loop-nest analysis.

* Review feedback.

* Review feedback.

* Review feedback.

* Use `Default`, not `std::default::Default`, as per @fitzgen

Co-authored-by: Nick Fitzgerald <fitzgen@gmail.com>

* Apply @fitzgen's comment elaboration to a doc-comment.

Co-authored-by: Nick Fitzgerald <fitzgen@gmail.com>

* Add stat for hitting the rewrite-depth limit.

* Some code motion in split prelude to make the diff a little clearer wrt `main`.

* Take @jameysharp's suggested `try_into()` usage for blockparam indices.

Co-authored-by: Jamey Sharp <jamey@minilop.net>

* Take @jameysharp's suggestion to avoid double-match on load op.

Co-authored-by: Jamey Sharp <jamey@minilop.net>

* Fix suggestion (add import).

* Review feedback.

* Fix stack_load handling.

* Remove redundant can_store case.

* Take @jameysharp's suggested improvement to FuncEGraph::build() logic

Co-authored-by: Jamey Sharp <jamey@minilop.net>

* Tweaks to FuncEGraph::build() on top of suggestion.

* Take @jameysharp's suggested clarified condition

Co-authored-by: Jamey Sharp <jamey@minilop.net>

* Clean up after suggestion (unused variable).

* Fix loop analysis.

* loop level asserts

* Revert constant-space loop analysis -- edge cases were incorrect, so let's go with the simple thing for now.

* Take @jameysharp's suggestion re: result_tys

Co-authored-by: Jamey Sharp <jamey@minilop.net>

* Fix up after suggestion

* Take @jameysharp's suggestion to use fold rather than reduce

Co-authored-by: Jamey Sharp <jamey@minilop.net>

* Fixup after suggestion

* Take @jameysharp's suggestion to remove elaborate_eclass_use's return value.

* Clarifying comment in terminator insts.

Co-authored-by: Jamey Sharp <jamey@minilop.net>
Co-authored-by: Nick Fitzgerald <fitzgen@gmail.com>

cranelift/filetests/filetests/egraph/remat.clif

@@ -0,0 +1,35 @@
+test optimize
+set opt_level=none
+set use_egraphs=true
+target x86_64
+
+function %f(i32) -> i32 {
+block0(v0: i32):
+    v1 = iconst.i32 42
+    v2 = iadd.i32 v0, v1
+    brnz v2, block1
+    jump block2
+
+block1:
+    v3 = iconst.i32 84
+    v4 = iadd.i32 v2, v3
+    return v4
+
+block2:
+    return v2
+}
+
+; check:  block0(v0: i32):
+; nextln:     v1 = iconst.i32 42
+; nextln:     v2 = iadd v0, v1
+; nextln:     brnz v2, block1
+; nextln:     jump block2
+; check:   block1:
+; nextln:     v5 = iconst.i32 126
+; nextln:     v6 = iadd.i32 v0, v5
+; nextln:     return v6
+; check:   block2:
+; nextln:     v3 = iconst.i32 42
+; nextln:     v4 = iadd.i32 v0, v3
+; nextln:     return v4
+