[DagCombiner] Allow shuffles to merge through bitcasts

Currently shuffles may only be combined if they are of the same type, despite the fact that bitcasts are often introduced in between shuffle nodes (e.g. x86 shuffle type widening).

This patch allows a single input shuffle to peek through bitcasts and if the input is another shuffle will merge them, shuffling using the smallest sized type, and re-applying the bitcasts at the inputs and output instead.

Dropped old ShuffleToZext test - this patch removes the use of the zext and vector-zext.ll covers these anyhow.

Differential Revision: http://reviews.llvm.org/D7939

llvm-svn: 231380