Correct removal of redundant moves

The logic for removing while iterating is non-standard and
a left over from a previous index based loop. This patch
replaces it with a standard erase based version.

This fixes a runtime crash with MSVC that invalidates the
iterator and then asserts. This also makes the code safe
in case the last move can be redundant.

Change-Id: Ie6990e0d65a3b83a4b7da3e2e89ed4e60a6cd215
Reviewed-on: https://chromium-review.googlesource.com/c/1488762Reviewed-by: Ben Titzer <titzer@chromium.org>
Commit-Queue: Ben Titzer <titzer@chromium.org>
Cr-Commit-Position: refs/heads/master@{#59868}

AUTHORS

@@ -46,6 +46,7 @@ Alexander Botero-Lowry <alexbl@FreeBSD.org>
 Alexander Karpinsky <homm86@gmail.com>
 Alexandre Vassalotti <avassalotti@gmail.com>
 Alexis Campailla <alexis@janeasystems.com>
+Allan Sandfeld Jensen <allan.jensen@qt.io>
 Amos Lim <eui-sang.lim@samsung.com>
 Andreas Anyuru <andreas.anyuru@gmail.com>
 Andrew Paprocki <andrew@ishiboo.com>

src/compiler/backend/gap-resolver.cc

@@ -96,8 +96,7 @@ void GapResolver::Resolve(ParallelMove* moves) {
   for (auto it = moves->begin(); it != moves->end();) {
     MoveOperands* move = *it;
     if (move->IsRedundant()) {
-      *it = moves->back();
-      moves->pop_back();
+      it = moves->erase(it);
       continue;
     }
     source_kinds.Add(GetKind(move->source()));