[DAGCombiner] Fold xor/and/or (bitcast(A), bitcast(B)) -> bitcast(op (A,B))...

[DAGCombiner] Fold xor/and/or (bitcast(A), bitcast(B)) -> bitcast(op (A,B)) anytime before LegalizeVectorOprs

xor/and/or (bitcast(A), bitcast(B)) -> bitcast(op (A,B)) was only being combined at the AfterLegalizeTypes stage, this patch permits the combine to occur anytime before then as well.

The main aim with this to improve the ability to recognise bitmasks that can be converted to shuffles.

I had to modify a number of AVX512 mask tests as the basic bitcast to/from scalar pattern was being stripped out, preventing testing of the mmask bitops. By replacing the bitcasts with loads we can get almost the same result.

Differential Revision: http://reviews.llvm.org/D18944

llvm-svn: 265998