DAG: Avoid OOB when legalizing vector indexing

DAG: Avoid OOB when legalizing vector indexing

If a vector index is out of bounds, the result is supposed to be
undefined but is not undefined behavior. Change the legalization
for indexing the vector on the stack so that an out of bounds
index does not create an out of bounds memory access.

git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@291604 91177308-0d34-0410-b5e6-96231b3b80d8