[X86] Remove ANY_EXTEND special case from canReduceVMulWidth

Removing this code doesn't affect any lit tests so it doesn't appear to be tested anymore. I assume it was when it was added, but I guess something else changed? Code coverage report also says its unused.

I mostly didn't like that it seemed to count the sign bits as if it was a sign_extend, but then set isPositive as if it was a zero_extend. It feels like we should have picked one interpretation?

Differential Revision: https://reviews.llvm.org/D54596

llvm-svn: 346995