Reland "uniform Ptr (UPtr) is a sub class of Ptr"

This is a reland of cef047a4904e8c8b4e3f8888c4fa19705eeec166

Fix strides in SkVMTest to be the right size.

Original change's description:
> uniform Ptr (UPtr) is a sub class of Ptr
>
> A pointer for a Uniform (UPtr) is a sub type of a Ptr. Everywhere you
> can use a Ptr a UPtr will work, but you can't use Ptr where you need
> a UPtr. All the UPtr instructions uniformF, gather32, etc are expected
> to be hoisted and therefore loaded only once. While the varyings
> instructions like load32, etc. are expected to remain in the body
> of the loop, and be evaluated each time through the loop.
>
> Change-Id: I4fe6458c2a4614872ed67cda1e81b05ea8a9e69e
> Reviewed-on: https://skia-review.googlesource.com/c/skia/+/436297
> Commit-Queue: Herb Derby <herb@google.com>
> Reviewed-by: Brian Osman <brianosman@google.com>

Change-Id: I858fa1224452ec801b6186fede353849edc895b5
Reviewed-on: https://skia-review.googlesource.com/c/skia/+/436564
Reviewed-by: Brian Osman <brianosman@google.com>
Commit-Queue: Herb Derby <herb@google.com>
7 files changed