[turbofan] support for Int64 in CheckedLoad/CheckedStore on 64-bit platforms.

[turbofan] support for Int64 in CheckedLoad/CheckedStore on 64-bit platforms.

This is to support WebAssembly 64-bit ints in the short term, since it
currently uses CheckedLoad/CheckedStore for accesses to the memory. In the
long run, we'll change this to be explicit bounds checks that throw on out
of bounds.

R=bmeurer@chromium.org
BUG=

Committed: https://crrev.com/963d664581e09d7846196b127f1fce845b9bc4a4
Cr-Commit-Position: refs/heads/master@{#30555}

[Benedikt Meurer, 2015-09-03 10:42:20]

lgtm

[titzer, 2015-09-03 10:43:43]

The CQ bit was checked by titzer@chromium.org

[commit-bot: I haz the power, 2015-09-03 10:43:48]

CQ is trying da patch. Follow status at
 https://chromium-cq-status.appspot.com/patch-status/1310323006/1
View timeline at
 https://chromium-cq-status.appspot.com/patch-timeline/1310323006/1

[commit-bot: I haz the power, 2015-09-03 10:45:42]

Committed patchset #1 (id:1)

[commit-bot: I haz the power, 2015-09-03 10:46:08]

Patchset 1 (id:??) landed as
https://crrev.com/963d664581e09d7846196b127f1fce845b9bc4a4
Cr-Commit-Position: refs/heads/master@{#30555}