[X86] Add DAG combine to turn (bitcast (and/or/xor (bitcast X), Y)) ->...

[X86] Add DAG combine to turn (bitcast (and/or/xor (bitcast X), Y)) -> (and/or/xor X, (bitcast Y)) when casting between GPRs and mask operations.

This reduces the number of transitions between k-registers and GPRs, reducing the number of instructions.

There's still some room for improvement to remove more transitions, but this is a good start.

llvm-svn: 324184