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=jarin@chromium.org

Review URL: https://codereview.chromium.org/1225993002

Cr-Commit-Position: refs/heads/master@{#29527}