[ARM64] [turbofan] Use 'mvn' rather than an equivalent 'orn'.

[ARM64] [turbofan] Use 'mvn' rather than an equivalent 'orn'.

`mvn xd, xn` is an architectural alias for `orn xd, xzr, xm`, so this
doesn't change code generation at all, but it's tidier and it addresses
a TODO.

BUG=

Committed: https://crrev.com/f88606e25dc45100bb2d3975f113e34d22f5e3d1
Cr-Commit-Position: refs/heads/master@{#28507}

[jbramley, 2015-05-20 10:38:13]

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

[Benedikt Meurer, 2015-05-20 10:41:26]

The CQ bit was checked by bmeurer@chromium.org

[Benedikt Meurer, 2015-05-20 10:41:26]

lgtm

[commit-bot: I haz the power, 2015-05-20 10:41:32]

CQ is trying da patch. Follow status at
 https://chromium-cq-status.appspot.com/patch-status/1151483002/1

[commit-bot: I haz the power, 2015-05-20 11:04:40]

Committed patchset #1 (id:1)

[commit-bot: I haz the power, 2015-05-20 11:05:00]

Patchset 1 (id:??) landed as
https://crrev.com/f88606e25dc45100bb2d3975f113e34d22f5e3d1
Cr-Commit-Position: refs/heads/master@{#28507}