[ARM64] Change TruncateInt64ToInt32 to a NOP.

[ARM64] Change TruncateInt64ToInt32 to a NOP.

BUG=

Committed: https://crrev.com/227353b76af72b7e3b8667c3ca80021fcdfe611c
Cr-Commit-Position: refs/heads/master@{#38612}

[georgia.kouveli, 2016-08-12 10:21:45]

georgia.kouveli@arm.com changed reviewers:
+ bmeurer@chromium.org, epertoso@chromium.org

[georgia.kouveli, 2016-08-12 10:21:45]

As far as I can tell, 32-bit operations don't rely on the top 32-bits in the
64-bit register having any particular value after the truncation from int64 to
int32, so we can replace the truncation with a NOP. This also under the
assumption that the result of TruncateInt64ToInt32 will not be used by 64-bit
operations directly (we'd zero- or sign-extend it with ChangeUint32ToUint64 or
ChangeInt32ToInt64 first).

This is in preparation of resurrecting patch set 1 of
https://codereview.chromium.org/2183923003. Matching the ASR with the truncate
here means we miss opportunities to match it with the 64-bit load instead (and
explains why we see LSR instructions after the 64-bit loads).

[epertoso, 2016-08-12 10:55:48]

lgtm

[georgia.kouveli, 2016-08-12 10:56:40]

The CQ bit was checked by georgia.kouveli@arm.com

[commit-bot: I haz the power, 2016-08-12 10:56:45]

CQ is trying da patch. Follow status at
 
https://chromium-cq-status.appspot.com/v2/patch-status/codereview.chromium.or...

[commit-bot: I haz the power, 2016-08-12 11:29:56]

Committed patchset #1 (id:1)

[commit-bot: I haz the power, 2016-08-12 11:30:09]

Description was changed from

==========
[ARM64] Change TruncateInt64ToInt32 to a NOP.

BUG=
==========

to

==========
[ARM64] Change TruncateInt64ToInt32 to a NOP.

BUG=

Committed: https://crrev.com/227353b76af72b7e3b8667c3ca80021fcdfe611c
Cr-Commit-Position: refs/heads/master@{#38612}
==========

[commit-bot: I haz the power, 2016-08-12 11:30:10]

Patchset 1 (id:??) landed as
https://crrev.com/227353b76af72b7e3b8667c3ca80021fcdfe611c
Cr-Commit-Position: refs/heads/master@{#38612}