Introduce TruncateTaggedToFloat64 and TruncateTaggedToWord32 into the
CodeStubAssembler, which encapsulates the ToNumber truncation and
returns the resulting number as either Float64 or further truncated to
Word32.

R=jarin@chromium.org
BUG=v8:4587
LOG=n

Review URL: https://codereview.chromium.org/1827813004

Cr-Commit-Position: refs/heads/master@{#35051}