impl load32

This means we can write a memset32 (load32 -> store32),
tested explicitly with the new unit test.

Slightly changes to the type protocol,
  - load and splat now generate scalars or vectors
    depending on how `scalar` is set
  - store should no longer have to pay attention to `scalar`;
    it's input values will already be the right size

Clean up some of the type declarations where we don't
actually need the subclass types, holding llvm::Type* instead.
This makes using ?: easier.

Change-Id: I2f98701ebdeead0513d355b2666b024794b90193
Reviewed-on: https://skia-review.googlesource.com/c/skia/+/273781
Reviewed-by: Herb Derby <herb@google.com>
Commit-Queue: Mike Klein <mtklein@google.com>
2 files changed