[x86] Fix the completely broken vector widening legalization of bswap.

This operation was classified as a binary operation in the widening
logic for some reason (clearly, untested). It is in fact a unary
operation. Add a RUN line to a test to exercise this for x86.

Note that again the vector widening strategy doesn't regress anything
and in one case removes a totally unecessary instruction that we
couldn't avoid when promoting the element type.