[X86] Correct 256 vpmovzx/vpmovsx isel patterns to check HasAVX2 instead of...

[X86] Correct 256 vpmovzx/vpmovsx isel patterns to check HasAVX2 instead of HasAVX to prevent fast-isel from using them incorrectly.

These are AVX2 instructions, but have been incorrectly marked in tablegen for a while. This wasn't a problem until r346784 switched the patterns to use target independent ISD opcodes. This made the patterns visible to fast isel.

Fixes PR39733

llvm-svn: 347375