Optimizations to egraph framework (#5391)

* Optimizations to egraph framework:

- Save elaborated results by canonical value, not latest value (union
  value). Previously we were artificially skipping and re-elaborating
  some values we already had because we were not finding them in the
  map.

- Make some changes to handling of icmp results: when icmp became
  I8-typed (when bools went away), many uses became `(uextend $I32 (icmp
  $I8 ...))`, and so patterns in lowering backends were no longer
  matching.

  This PR includes an x64-specific change to match `(brz (uextend (icmp
  ...)))` and similarly for `brnz`, but it also takes advantage of the
  ability to write rules easily in the egraph mid-end to rewrite selects
  with icmp inputs appropriately.

- Extend constprop to understand selects in the egraph mid-end.

With these changes, bz2.wasm sees a ~1% speedup, and spidermonkey.wasm
with a fib.js input sees a 16.8% speedup:

```
$ time taskset 1 target/release/wasmtime run --allow-precompiled --dir=. ./spidermonkey.base.cwasm ./fib.js
1346269
taskset 1 target/release/wasmtime run --allow-precompiled --dir=.  ./fib.js  2.14s user 0.01s system 99% cpu 2.148 total
$ time taskset 1 target/release/wasmtime run --allow-precompiled --dir=. ./spidermonkey.egraphs.cwasm ./fib.js
1346269
taskset 1 target/release/wasmtime run --allow-precompiled --dir=.  ./fib.js  1.78s user 0.01s system 99% cpu 1.788 total
```

* Review feedback.

cranelift/codegen/src/opts/algebraic.isle

@@ -185,3 +185,25 @@
       (remat x))
 (rule (simplify x @ (f64const _ _))
       (remat x))
+
+;; Optimize icmp-of-icmp.
+(rule (simplify (icmp ty
+                      (IntCC.NotEqual)
+                      (uextend _ inner @ (icmp ty _ _ _))
+                      (iconst _ (u64_from_imm64 0))))
+      (subsume inner))
+
+(rule (simplify (icmp ty
+                      (IntCC.Equal)
+                      (uextend _ (icmp ty cc x y))
+                      (iconst _ (u64_from_imm64 0))))
+      (subsume (icmp ty (intcc_inverse cc) x y)))
+
+;; Optimize select-of-uextend-of-icmp to select-of-icmp, because
+;; select can take an I8 condition too.
+(rule (simplify
+       (select ty (uextend _ c @ (icmp _ _ _ _)) x y))
+      (select ty c x y))
+(rule (simplify
+       (select ty (uextend _ c @ (icmp _ _ _ _)) x y))
+      (select ty c x y))

cranelift/codegen/src/opts/cprop.isle

@@ -130,5 +130,12 @@
        (bxor ty (bxor ty x k1 @ (iconst ty _)) k2 @ (iconst ty _)))
       (bxor ty x (bxor ty k1 k2)))
 
+(rule (simplify
+       (select ty (iconst _ (u64_from_imm64 (u64_nonzero _))) x y))
+      x)
+(rule (simplify
+       (select ty (iconst _ (u64_from_imm64 0)) x y))
+      y)
+
 ;; TODO: fadd, fsub, fmul, fdiv, fneg, fabs