NativeRegExpMacroAssembler::Match() can return either a Result sentinel
or an int indicating the number of matches, so it should return a plain
int which we can only safely cast to Result or IrregexpResult when it's
guaranteed to be the former case.

Bug: v8:3770
Change-Id: I4c3447e0cdebd5f825964e086574ab504a1799cd
Reviewed-on: https://chromium-review.googlesource.com/c/1435735Reviewed-by: Yang Guo <yangguo@chromium.org>
Commit-Queue: Jakob Kummerow <jkummerow@chromium.org>
Cr-Commit-Position: refs/heads/master@{#59142}