[X86] Change X86 backend to look for 'min-legal-vector-width' attribute instead of...

[X86] Change X86 backend to look for 'min-legal-vector-width' attribute instead of 'required-vector-width' when determining whether 512-bit vectors should be legal.

The required-vector-width attribute was only used for backend testing and has never been generated by clang.

I believe clang is now generating min-legal-vector-width for vector uses in user code.

With this I believe passing -mprefer-vector-width=256 to clang should prevent use of zmm registers in the generated assembly unless the user used a 512-bit intrinsic in their source code.

llvm-svn: 345317