port e06f7d78 (r33347)

  original commit message:
  The new operator converts an int32 input to float32. If the input cannot
  be represented exactly in float32, the value is rounded using the
  round-ties-even rounding mode (the default rounding mode).

  I provide implementations of the new operator for x64, ia32, arm, arm64,
  mips, mips64, ppc, and ppc64.

BUG=

Review URL: https://codereview.chromium.org/1597403002

Cr-Commit-Position: refs/heads/master@{#33351}