For sequential, fully-in-bound vectors:
* Loads can safely be a regular vector load. We mask just to keep behavior consistent with rr::MaskedLoad and rr::Gather.
* Stores of non-atomics can be implemented as a read-modify-write.

These optimizations have drastic performance improvements on architectures where there are no masked-read and masked-write instructions.

Bug: b/135609394
Change-Id: I552cc38f4aeae73f8db079a0a11da6a8db857710
Reviewed-on: https://swiftshader-review.googlesource.com/c/SwiftShader/+/33628Tested-by: Ben Clayton <bclayton@google.com>
Kokoro-Presubmit: kokoro <noreply+kokoro@google.com>
Reviewed-by: Nicolas Capens <nicolascapens@google.com>