It's required by the spec -- and observable -- that the index be validated
before the conversion of the value(s) via ToInteger.

The previous implementation also had an old test for validating the atomic
index, which has now been switched to ToIndex.

This also exposed an issue in the ia32 code generator: cmpxchg_b requires a
byte register, but the ia32 instruction selector was ensuring that the
new_value was a byte register, not the TempRegister. This change forces the
temp register to use edx, which always can be used as a byte register (dl).
This is the same behavior as currently used in UseByteRegister.

BUG=v8:4614
R=jarin@chromium.org,jkummerow@chromium.org

Review-Url: https://codereview.chromium.org/2814753003
Cr-Commit-Position: refs/heads/master@{#44626}