[x86] enable storeOfVectorConstantIsCheap() target hook

This allows vector-sized store merging of constants in DAGCombiner using the existing code in MergeConsecutiveStores(). 
All of the twisted logic that decides exactly what vector operations are legal and fast for each particular CPU are 
handled separately in there using the appropriate hooks.

For the motivating tests in merge-store-constants.ll, we already produce the same vector code in IR via the SLP vectorizer. 
So this is just providing a backend backstop for code that doesn't go through that pass (-O1). More details in PR24449:
https://bugs.llvm.org/show_bug.cgi?id=24449 (this change should be the last step to resolve that bug)

Differential Revision: https://reviews.llvm.org/D37451

llvm-svn: 313458