add 64-bit load/store ops

This adds load/store ops for 64-bit values, with two load64 instructions
returning the low and high 32-bits each, and store64 taking both.

These are implemented in the interpreter and tested but not yet JIT'd
or hooked up for loading and storing 64-bit PixelFormats.  Hopefully
those two CLs to follow shortly.

Change-Id: I7e5fc3f0ee5a421adc9fb355d0b6b661f424b505
Reviewed-on: https://skia-review.googlesource.com/c/skia/+/303380
Reviewed-by: Herb Derby <herb@google.com>
Commit-Queue: Mike Klein <mtklein@google.com>
4 files changed