[DAGCombiner] Add a peekThroughBitcast to MergeStoresOfConstantsOrVecElts to...

[DAGCombiner] Add a peekThroughBitcast to MergeStoresOfConstantsOrVecElts to fix a crash if we are storing a bitcast of a constant.

Loading a constant into a k-register in AVX512 requires a bitcast from a scalar constant. In the test case here we have a k-register store that gets split into multiple parts of KNL. MergeConsecutiveStores sees each of these pieces as a consecutive store and looks through the bitcast to find the underly scalar constant. But when we went to create the combined store we didn't look through the same bitcast.

llvm-svn: 326677