unroll STORE_3 and STORE_4

Again I noticed that we could get better codegen in
store_888, store_161616, and store_fff, and I don't
think it's terribly more error-prone.

Change-Id: I9b2391954f0342996d8bf169ed2e3a8907c19629
Reviewed-on: https://skia-review.googlesource.com/96960
Reviewed-by: Brian Osman <brianosman@google.com>
1 file changed