[DAG] Support store merging of vector constant stores (try 2)

Original commit didn't handle the case where one of the stores was a
truncating store of the build_vector.  The existing codepath produced
wrong code (which thankfully also failed asserts) instead of guarding
against unexpected types.  Original commit message follows..

Ran across this when making a change to RISCV memset lowering. Seems
very odd that manually merging a store into a vector prevents it from
being further merged.

Differential Revision: https://reviews.llvm.org/D156349