Rewrite DCE pass (#3274)

The DCE pass is one of the oldest in binaryen, and had quite a lot of
cruft from the changes in unreachability and other stuff in wasm and
binaryen's history. This PR rewrites it from scratch, making it about
1/3 the size.

I noticed this when looking for places to use code autogeneration.
The old version had annoying boilerplate, while the new one avoids
any need for it.

There may be noticeable differences, as the old pass did more than
it needed to. It overlapped with remove-unused-names for some
reason I don't remember. The new pass leaves that to the other
pass to do. I added another run of remove-unused-names to avoid
noticeable differences in optimized builds, but you can see
differences in the testcases that only run DCE by itself. (The test
differences in this PR are mostly whitespace.)

(The overlap is that if a block ended up not needed, that is, all
branches to it were removed, the old DCE would remove the block.)

This pass is about 15% faster than the old version. However, when
adding another run of remove-unused-names the difference
basically vanishes, so this isn't a speedup.

1 files changed, 14 insertions, 8 deletions

diff --git a/test/wasm2js/br.2asm.js b/test/wasm2js/br.2asm.js
index 73484e457..c60de9fc2 100644
--- a/test/wasm2js/br.2asm.js
+++ b/test/wasm2js/br.2asm.js
@@ -106,8 +106,10 @@ function asmFunc(global, env) {
  function $13() {
   var $0 = 0, $1_1 = 0, $3_1 = 0;
   block : {
-   $0 = 3;
-   break block;
+   loop_in : while (1) {
+    $0 = 3;
+    break block;
+   };
   }
   return $0 | 0;
  }
@@ -115,9 +117,11 @@ function asmFunc(global, env) {
  function $14() {
   var $0 = 0, $1_1 = 0, $3_1 = 0;
   block : {
-   dummy();
-   $0 = 4;
-   break block;
+   loop_in : while (1) {
+    dummy();
+    $0 = 4;
+    break block;
+   };
   }
   return $0 | 0;
  }
@@ -125,9 +129,11 @@ function asmFunc(global, env) {
  function $15() {
   var $0 = 0;
   block : {
-   dummy();
-   $0 = 5;
-   break block;
+   loop_in : while (1) {
+    dummy();
+    $0 = 5;
+    break block;
+   };
   }
   return $0 | 0;
  }