This reverts commit 5616f91a.

Reason for revert: Tanks performance on codeload/ia32 by >1%

Original change's description:
> [parser] Use memcmp for keyword checks
> 
> Small readability increase for the keyword check magic, using memcmp
> instead of a chain of raw comparisons. Could allow better codegen for
> memcmp-aware compilers, though in practice seems to have little effect
> on generated code.
> 
> Change-Id: I91020fe67cebc9270c61c4c678e15217e436afff
> Reviewed-on: https://chromium-review.googlesource.com/c/1340291
> Reviewed-by: Toon Verwaest <verwaest@chromium.org>
> Commit-Queue: Leszek Swirski <leszeks@chromium.org>
> Cr-Commit-Position: refs/heads/master@{#57595}

TBR=leszeks@chromium.org,verwaest@chromium.org

# Not skipping CQ checks because original CL landed > 1 day ago.

Change-Id: Ifd4070efb3e692eb70e5d8b3adb04d7d7e45ef0e
Reviewed-on: https://chromium-review.googlesource.com/c/1345994Reviewed-by: Toon Verwaest <verwaest@chromium.org>
Reviewed-by: Leszek Swirski <leszeks@chromium.org>
Commit-Queue: Toon Verwaest <verwaest@chromium.org>
Cr-Commit-Position: refs/heads/master@{#57665}