[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

llvm-svn: 235578