Cranelift: GVN spectre guards and run redundant load elimination twice (#5517)

* Cranelift: Make spectre guards GVN-able

While these instructions have a side effect that is otherwise invisible to the
optimizer, the side effect in question is idempotent, so it can be de-duplicated
by GVN.

* Cranelift: Run redundant load replacement and GVN twice

This allows us to actually replace redundant Wasm loads with dynamic memories.

While this improves our hand-crafted test sequences, it doesn't seem to have any
improvement on sightglass benchmarks run with dynamic memories, however it also
isn't a hit to compilation times, so seems generally good to land anyways:

```
$ cargo run --release -- benchmark -e ~/scratch/once.so -e ~/scratch/twice.so -m insts-retired --processes 20 --iterations-per-process 3 --engine-flags="--static-memory-maximum-size 0" -- benchmarks/default.suite
compilation :: instructions-retired :: benchmarks/spidermonkey/benchmark.wasm

  No difference in performance.

  [683595240 683768610.53 684097577] once.so
  [683597068 700115966.83 1664907164] twice.so

instantiation :: instructions-retired :: benchmarks/spidermonkey/benchmark.wasm

  No difference in performance.

  [44107 60411.07 92785] once.so
  [44138 59552.32 92097] twice.so

compilation :: instructions-retired :: benchmarks/bz2/benchmark.wasm

  No difference in performance.

  [17369916 17404839.78 17471458] once.so
  [17369935 17625713.87 30700150] twice.so

compilation :: instructions-retired :: benchmarks/pulldown-cmark/benchmark.wasm

  No difference in performance.

  [126523640 126566170.80 126648265] once.so
  [126523076 127174580.30 163145149] twice.so

instantiation :: instructions-retired :: benchmarks/pulldown-cmark/benchmark.wasm

  No difference in performance.

  [34569 35686.25 36513] once.so
  [34651 35749.97 36953] twice.so

instantiation :: instructions-retired :: benchmarks/bz2/benchmark.wasm

  No difference in performance.

  [35146 36639.10 37707] once.so
  [34472 36580.82 38431] twice.so

execution :: instructions-retired :: benchmarks/spidermonkey/benchmark.wasm

  No difference in performance.

  [7055720115 7055841324.82 7056180024] once.so
  [7055717681 7055877095.85 7056225217] twice.so

execution :: instructions-retired :: benchmarks/pulldown-cmark/benchmark.wasm

  No difference in performance.

  [46436881 46437081.28 46437691] once.so
  [46436883 46437127.68 46437766] twice.so

execution :: instructions-retired :: benchmarks/bz2/benchmark.wasm

  No difference in performance.

  [653010530 653010533.27 653010539] once.so
  [653010531 653010532.95 653010538] twice.so
```

cranelift/codegen/meta/src/gen_inst.rs

@@ -512,6 +512,13 @@ fn gen_opcodes(all_inst: &AllInstructions, fmt: &mut Formatter) {
             "Does this instruction have other side effects besides can_* flags?",
             fmt,
         );
+        gen_bool_accessor(
+            all_inst,
+            |inst| inst.side_effects_okay_for_gvn,
+            "side_effects_okay_for_gvn",
+            "Despite `self.other_side_effects() == true`, is this instruction okay to GVN?",
+            fmt,
+        )
     });
     fmt.line("}");
     fmt.empty_line();