Add target hook to allow merging stores of nonzero constants

Add target hook to allow merging stores of nonzero constants

On GPU targets, materializing constants is cheap and stores are
expensive, so only doing this for zero vectors was silly.

Most of the new testcases aren't optimally merged, and are for
later improvements.

git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@238108 91177308-0d34-0410-b5e6-96231b3b80d8