[X86] Add test cases to show that we don't properly take...

[X86] Add test cases to show that we don't properly take -mprefer-vector-width=256 and -min-legal-vector-width=256 into account when costing sext/zext.

The check lines marked AVX256 in the zext256/sext256 functions should be closer to the AVX values which would take into account a splitting cost.

llvm-svn: 347722