more symmetry for from_half/to_half

Tweaks to make the parallels between from_half and to_half stand out.

We can logically do the `auto denorm = em < ...;` comparisons as either
U32 or I32.  U32 would read more naturally, but we do I32 because some
instruction sets have direct signed comparison but must synthesize an
unsigned comparison.

CQ_INCLUDE_TRYBOTS=skia.primary:Test-Android-Clang-PixelC-CPU-TegraX1-arm64-Release-Android,Test-Android-Clang-Ci20-CPU-IngenicJZ4780-mipsel-Release-Android,Test-Android-Clang-Nexus10-CPU-Exynos5250-arm-Release-Android,Test-Mac-Clang-MacMini6.2-CPU-AVX-x86_64-Release,Test-Ubuntu-GCC-GCE-CPU-AVX2-x86-Debug,Test-Ubuntu-GCC-GCE-CPU-AVX2-x86_64-Debug

Change-Id: Ic74fe5b3b850f5bb7fd00fd4435bc32b8628eecd
Reviewed-on: https://skia-review.googlesource.com/13963
Reviewed-by: Matt Sarett <msarett@google.com>
Commit-Queue: Mike Klein <mtklein@chromium.org>
3 files changed