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, 12 insertions, 30 deletions

diff --git a/test/example/c-api-unused-mem.txt b/test/example/c-api-unused-mem.txt
index 6e67735bd..3c2301ace 100644
--- a/test/example/c-api-unused-mem.txt
+++ b/test/example/c-api-unused-mem.txt
@@ -9,32 +9,23 @@
   (local $0 i32)
   (local $1 i32)
   (local $2 i64)
-  (block $block$1$break
+  (block $block$2$break
    (set_local $0
     (i32.load
      (i32.const 0)
     )
    )
    (block
-    (br $block$1$break)
+    (br $block$2$break)
    )
   )
   (block
-   (block $block$2$break
-    (block $bb0
-    )
-    (block
-     (br $block$2$break)
-    )
-   )
    (block
-    (block $bb1
-     (i32.store
-      (i32.const 0)
-      (get_local $0)
-     )
-     (return)
+    (i32.store
+     (i32.const 0)
+     (get_local $0)
     )
+    (return)
    )
   )
  )
@@ -46,7 +37,7 @@
   (call $main)
  )
 )
-169
+151
 (module
  (type $0 (func))
  (type $1 (func))
@@ -70,22 +61,13 @@
   )
   (block $label$3
    (block $label$4
-    (block $label$5
-    )
-    (block $label$6
-     (br $label$4)
-    )
-   )
-   (block $label$7
-    (block $label$8
-     (i32.store
-      (i32.const 0)
-      (get_local $0)
-     )
-     (return)
+    (i32.store
+     (i32.const 0)
+     (get_local $0)
     )
-    (unreachable)
+    (return)
    )
+   (unreachable)
   )
  )
  (func $__wasm_start (; 1 ;) (type $1)