Relooper CFG optimizations (#1759)

Previously the relooper would do some optimizations when deciding when to use an if vs a switch, how to group blocks, etc. This PR adds an additional pre-optimization phase with some basic but useful simplify-cfg style passes,

* Skip empty blocks when they have just one exit.
* Merge exiting branches when they are equivalent.
* Canonicalize block contents to make such comparisons more useful.
* Turn a trivial one-target switch into a simple branch.

This can help in noticeable ways when running the rereloop pass, e.g. on LLVM wasm backend output.

Also:

* Binaryen C API changes to the relooper, which now gets a Module for its constructor. It needs it for the optimizations, as it may construct new nodes.
* Many relooper-fuzzer improvements.
* Clean up HashType usage.

1 files changed, 2 insertions, 2 deletions

diff --git a/src/ir/hashed.h b/src/ir/hashed.h
index b15342aba..61bf19478 100644
--- a/src/ir/hashed.h
+++ b/src/ir/hashed.h
@@ -26,7 +26,7 @@ namespace wasm {
 // An expression with a cached hash value
 struct HashedExpression {
   Expression* expr;
-  size_t hash;
+  HashType hash;
 
   HashedExpression(Expression* expr) : expr(expr) {
     if (expr) {
@@ -38,7 +38,7 @@ struct HashedExpression {
 };
 
 struct ExpressionHasher {
-  size_t operator()(const HashedExpression value) const {
+  HashType operator()(const HashedExpression value) const {
     return value.hash;
   }
 };