[X86] Improve combineCastedMaskArithmetic to fold (bitcast (vXi1 (and/or/xor...

[X86] Improve combineCastedMaskArithmetic to fold (bitcast (vXi1 (and/or/xor X, C)))->(vXi1 (and/or/xor (bitcast X), (bitcast C)) where C is a constant build_vector.

Most vxi1 constant build vectors have to be implemented in the scalar domain anyway so we'll probably end up with a cast there later. But by then its too late to do the combine to get rid of it.

llvm-svn: 324662