[AArch64] Fix bugs in isel lowering replaceSplatVectorStore.

Summary:
Fix off-by-one indexing error in loop checking that inserted value was a
splat vector.

Add code to check that INSERT_VECTOR_ELT nodes constructing the splat
vector have the expected constant index values.

Reviewers: t.p.northover, jmolloy, mcrosier

Subscribers: aemerson, llvm-commits, rengolin

Differential Revision: https://reviews.llvm.org/D26409

llvm-svn: 286616