[ARM64] [turbofan] Improve construction of doubles.

[ARM64] [turbofan] Improve construction of doubles.

Improve the code generated for construction of a 64-bit floating point
number from two 32-bit integers.

Previously, this moved FP->core, inserted, then moved core->FP for each
half. Now, we construct the double in an X register and move core->FP.
Typically, the temporary register aliases the input register, so the
sequence improves from six to two instructions.

Patch from Martyn Capewell <m.m.capewell@googlemail.com>;.

BUG=

Committed: https://crrev.com/12993637118b25194642666db6c3ce44d9052d94
Cr-Commit-Position: refs/heads/master@{#27227}

[jbramley, 2015-03-16 14:43:03]

jacob.bramley@arm.com changed reviewers:
+ bmeurer@chromium.org, ulan@chromium.org

[Benedikt Meurer, 2015-03-16 16:36:09]

Love it, LGTM!

[Benedikt Meurer, 2015-03-16 16:36:14]

The CQ bit was checked by bmeurer@chromium.org

[Benedikt Meurer, 2015-03-16 16:36:18]

The CQ bit was unchecked by bmeurer@chromium.org

[jbramley, 2015-03-16 16:53:12]

The CQ bit was checked by jacob.bramley@arm.com

[commit-bot: I haz the power, 2015-03-16 16:53:23]

CQ is trying da patch. Follow status at
 https://chromium-cq-status.appspot.com/patch-status/1008003004/1

[commit-bot: I haz the power, 2015-03-16 17:15:21]

Committed patchset #1 (id:1)

[commit-bot: I haz the power, 2015-03-16 17:15:35]

Patchset 1 (id:??) landed as
https://crrev.com/12993637118b25194642666db6c3ce44d9052d94
Cr-Commit-Position: refs/heads/master@{#27227}