We had failed to enable these implementations for the
`ExtendedMemOperand` helpers. We had already implemented the non-helper
forms, which are already tested in CI. These helpers just weren't
updated?
Noticed this when running libaom's SSE4.1 tests, where it managed to
execute a pmovzxbq instruction with reg+reg memory source and was
breaking the test results.
There are /very/ few vector register operations that access only 8-bit
or 16-bit in vectors so this flew under the radar for quite a while.
Fixes their unit tests.
Also adds a unittest using sse4.1 pmovzxbq to ensure we support the
reg+reg case, and also a few other instructions to test 8-bit and 16-bit
vector loads and stores.
