SIMD.js potentially adds to the standard library passed into
asm.js modules. Splitting off the point where the SIMD object
would be referenced to allow work on SIMD typing to occur orthogonally.

Adding VariableInfo to allow tracking of simd constructors / check functions. Using this for fround.

BUG= https://code.google.com/p/v8/issues/detail?id=4203
TEST=test-asm-validator
R=titzer@chromium.org,aseemgarg@chromium.org
LOG=N

Looking at simd.js

Review URL: https://codereview.chromium.org/1473513004

Cr-Commit-Position: refs/heads/master@{#32431}