[AArch64] Fix PR32384: bump up the number of stores per memset and memcpy

As suggested in https://bugs.llvm.org/show_bug.cgi?id=32384#c1, this change
makes the inlining of `memset()` and `memcpy()` more aggressive when
compiling for speed.  The tuning remains the same when optimizing for size.

Patch by: Sebastian Pop <s.pop@samsung.com>
          Evandro Menezes <e.menezes@samsung.com>

Differential revision: https://reviews.llvm.org/D45098

llvm-svn: 333429