[DAGCombiner] Remove extra bitcasts surrounding vector shuffles 

Patch to remove extra bitcasts from shuffles, this is often a legacy of XformToShuffleWithZero being used to combine bitmaskings (of float vectors bitcast to integer vectors) into shuffles: bitcast(shuffle(bitcast(s0),bitcast(s1))) -> shuffle(s0,s1)

Differential Revision: http://reviews.llvm.org/D9097

git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@235578 91177308-0d34-0410-b5e6-96231b3b80d8
2 files changed