port 4b38c158 (r29527).

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.

BUG=

Review URL: https://codereview.chromium.org/1227923003

Cr-Commit-Position: refs/heads/master@{#29629}