[X86] Remove avx512 isel patterns for fpextend+load. Prefer to only match fp extloads instead.

DAG combine will usually fold fpextend+load to an fp extload anyway. So the
256 and 512 patterns were probably unnecessary. The 128 bit pattern was special
in that it looked for a v4f32 load, but then used it in an instruction that
only loads 64-bits. This is bad if the load happens to be volatile. We could
probably make the patterns volatile aware, but that's more work for something
that's probably rare. The peephole pass might kick in and save us anyway. We
might also be able to fix this with some additional DAG combines.

This also adds patterns for vselect+extload to enabled masked vcvtps2pd to be
used. Previously we looked for the unlikely vselect+fpextend+load.

llvm-svn: 362199