For the old asm.js pipeline, there's no point in running either the
JSCallReducer or the JSNativeContextSpecialization, as both will not
kick in for any asm.js relevant code.

So remove the kDeoptimizationEnabled flag from both of them and just
run them on regular JavaScript only.

Drive-by-fix: Slightly rearrange keyed access to String receivers in the
JSNativeContextSpecialization::ReduceKeyedAccess method to access constant
characters independent of the IC mode (as long as the constant index is
within the range of the receiver).

BUG=v8:5267
R=jarin@chromium.org

Review-Url: https://codereview.chromium.org/2743253002
Cr-Commit-Position: refs/heads/master@{#43813}