The TruncateFloat32ToInt64 operator converts a float32 to an int64 using
the round-to-zero rounding mode (truncate). If the input value is
outside the int64 range, then the result depends on the architecture. I
implemented the operator on x64, arm64, and mips64.

R=titzer@chromium.org, jacob.bramley@arm.com

Review URL: https://codereview.chromium.org/1476063002

Cr-Commit-Position: refs/heads/master@{#32315}