Convert image width and height (used by tiling) to float once.

Convert image width and height (used by tiling) to float once.

The storage cost is the same, so might as well do this when building the pipeline instead of when running it.  This also avoids the awkward cvtsi2ss instruction that screws with register renaming.

CQ_INCLUDE_TRYBOTS=skia.primary:Test-Ubuntu-GCC-GCE-CPU-AVX2-x86_64-Release-SKNX_NO_SIMD

Change-Id: I1c7d5bad558870256a31e3da969eee5d80fb93a8
Reviewed-on: https://skia-review.googlesource.com/5782
Reviewed-by: Herb Derby <herb@google.com>
Commit-Queue: Mike Klein <mtklein@chromium.org>