Fix stack indexing bug in vectorized unsigned -> float conversion

Also add a test of all three conversions (scalar & vector).

Change-Id: I1671e992760ad538304d2602e37cd9792a49e9cb
Reviewed-on: https://skia-review.googlesource.com/c/skia/+/213660
Reviewed-by: Mike Klein <mtklein@google.com>
Commit-Queue: Brian Osman <brianosman@google.com>
2 files changed