[X86] Remove some seemingly unnecessary patterns that supported vector...

[X86] Remove some seemingly unnecessary patterns that supported vector zext/sext with 256-bit source types producing a 256-bit result.

These patterns just extracted the source down to 128-bits to use the instructions. AVX512 seems to have blindly copied them over for VLX, but did not create similar patterns for 512-bit sources. So I'm hoping the backend can't actually produce these cases.

llvm-svn: 275240