PPC: [turbofan] Add TruncationMode for TruncateFloat64ToInt32.

PPC: [turbofan] Add TruncationMode for TruncateFloat64ToInt32.

Port 4b38c15817033ccd9a65efbb3d038ae2423293c2

Original commit message:
    We actually need round to zero truncation to implement the counterpart
    of LDoubleToI in TurboFan, which tries to convert a double to an integer
    as required for keyed load/store optimizations.

    Drive-by-cleanup: Reduce some code duplication in the InstructionSelector
    implementations.

R=bmeurer@chromium.org, dstence@us.ibm.com, michael_dawson@ca.ibm.com
BUG=

Committed: https://crrev.com/2798e118bd8b0819c333f5d4907d979195498f07
Cr-Commit-Position: refs/heads/master@{#29533}

[dstence, 2015-07-08 13:29:20]

lgtm

[MTBrandyberry, 2015-07-08 13:30:20]

The CQ bit was checked by mbrandy@us.ibm.com

[commit-bot: I haz the power, 2015-07-08 13:30:32]

CQ is trying da patch. Follow status at
 https://chromium-cq-status.appspot.com/patch-status/1220373003/1

[commit-bot: I haz the power, 2015-07-08 13:55:00]

Committed patchset #1 (id:1)

[commit-bot: I haz the power, 2015-07-08 13:55:17]

Patchset 1 (id:??) landed as
https://crrev.com/2798e118bd8b0819c333f5d4907d979195498f07
Cr-Commit-Position: refs/heads/master@{#29533}