ARM64: improve non-zero memset isel by ~2x

Summary:
I added a few ARM64 memset codegen tests in r341406 and r341493, and annotated
where the generated code was bad. This patch fixes the majority of the issues by
requesting that a 2xi64 vector be used for memset of 32 bytes and above.

The patch leaves the former request for f128 unchanged, despite f128
materialization being suboptimal: doing otherwise runs into other asserts in
isel and makes this patch too broad.

This patch hides the issue that was present in bzero_40_stack and bzero_72_stack
because the code now generates in a better order which doesn't have the store
offset issue. I'm not aware of that issue appearing elsewhere at the moment.

<rdar://problem/44157755>

Reviewers: t.p.northover, MatzeB, javed.absar

Subscribers: eraman, kristof.beyls, chrib, dexonsmith, llvm-commits

Differential Revision: https://reviews.llvm.org/D51706

llvm-svn: 341558