[X86] Add test cases to show incorrect use of a 512 bit vector in v32i8...

[X86] Add test cases to show incorrect use of a 512 bit vector in v32i8 multiply lowering with prefer-vector-width=256.

On the min-legal-vector-width test this actually causes some of the v32i16 operations we emitted to be scalarized.

llvm-svn: 347130