ARM64: Support sign extend for add and subtract

ARM64: Support sign extend for add and subtract

Support sxtb and sxth extend operators on add and subtract, as we've
done for ubtx/h. This is similar to ARM support for sxtab/h.

BUG=

Committed: https://crrev.com/8e846a61660ae849a0e6cc8a43376a97f412927b
Cr-Commit-Position: refs/heads/master@{#27624}

[martyn.capewell, 2015-04-07 09:34:07]

martyn.capewell@arm.com changed reviewers:
+ bmeurer@chromium.org, ulan@chromium.org

[Benedikt Meurer, 2015-04-07 10:20:40]

Nice. LGTM

[martyn.capewell, 2015-04-07 11:07:27]

The CQ bit was checked by martyn.capewell@arm.com

[commit-bot: I haz the power, 2015-04-07 11:07:32]

CQ is trying da patch. Follow status at
 https://chromium-cq-status.appspot.com/patch-status/1064813003/1

[commit-bot: I haz the power, 2015-04-07 11:34:37]

Committed patchset #1 (id:1)

[commit-bot: I haz the power, 2015-04-07 11:34:52]

Patchset 1 (id:??) landed as
https://crrev.com/8e846a61660ae849a0e6cc8a43376a97f412927b
Cr-Commit-Position: refs/heads/master@{#27624}