[X86] Add test case for a masked load mask becoming all ones after type legalization.

We should be able to turn this into a unmasked load. X86 has an
optimization to detect that the first and last element aren't masked
and then turn the whole thing into an unmasked load and a blend.
That transform is disabled on avx512 though.

But if we know the blend isn't needed, then the unmasked load by
itself should always be profitable.