[ia32] Require SSE2.

[ia32] Require SSE2.

This way we stay in sync with what Chromium does and we reduce the
number of bugs caused by 80-to-{32,64}-bit rounding errors. It's also
a requirement for switching to signaling NaN for FixedDoubleArray holes.

R=jkummerow@chromium.org

Review URL: https://codereview.chromium.org/864483002

Cr-Commit-Position: refs/heads/master@{#26154}