[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

Committed: https://crrev.com/655b112466e3d9138e07f78188f4fc5fdc16980c
Cr-Commit-Position: refs/heads/master@{#26154}

[Jakob Kummerow, 2015-01-20 12:17:05]

LGTM.

I don't think the big comment adds much value (we know what SSE2 is), but if you
like it, you can keep it.

[Benedikt Meurer, 2015-01-20 12:23:52]

The CQ bit was checked by bmeurer@chromium.org

[commit-bot: I haz the power, 2015-01-20 12:24:28]

CQ is trying da patch. Follow status at
 https://chromium-cq-status.appspot.com/patch-status/864483002/1

[commit-bot: I haz the power, 2015-01-20 12:53:49]

Committed patchset #1 (id:1)

[commit-bot: I haz the power, 2015-01-20 12:53:59]

Patchset 1 (id:??) landed as
https://crrev.com/655b112466e3d9138e07f78188f4fc5fdc16980c
Cr-Commit-Position: refs/heads/master@{#26154}