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=

Review URL: https://codereview.chromium.org/1220373003

Cr-Commit-Position: 972c6d2dc6dd5efdad1377c0d224e03eb8f276f7@{#29533}

UniversalFuture/moosh,UniversalFuture/moosh,UniversalFuture/moosh,UniversalFuture/moosh