NFC: expand memset inline arm64 coverage

NFC: expand memset inline arm64 coverage

I'm looking at some codegen optimization in this area and want to make sure I understand the current codegen and don't regress it. This patch simply expands the two existing tests to capture more of the current code generation when it comes to heap-based and stack-based small memset on arm64. The tested code is already pretty good, notably when it comes to using STP, FP stores, FP immediate generation, and folding one of the stores into a stack spill when possible. The uses of STUR could be improved, and some more pairing could occur. Straying from bzero patterns currently yield suboptimal code, and I expect a variety of small changes could make things way better.

git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@341406 91177308-0d34-0410-b5e6-96231b3b80d8