Merging r367898:

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r367898 | evandro | 2019-08-05 20:09:14 +0200 (Mon, 05 Aug 2019) | 22 lines

[AArch64] Expand bcmp() for small block lengths

Patch D56593 by @courbet results in calls to `bcmp()` in some cases, should
the target support the it.  Unless `TTI::MemCmpExpansionOptions()`
is overridden by the target.

In a proprietary benchmark we see a performance drop of about 12% on PNG
compression before this patch, though it passes all tests.

This patch mirrors X86 for AArch64 and initializes
`TTI::MemCmpExpansionOptions()` to then expand calls to `bcmp()` when
appropriate.  No tuning of the parameters was performed, but, at this point,
it's enough to recover the performance drop above.

This problem also exists on ARM.  Once a consensus is reached for AArch64, we
can work to fix ARM as well.

Authors:
- Evandro Menezes (@evandro) <e.menezes@samsung.com>
- Brian Rzycki (@brzycki) <b.rzycki@samsung.com>

Differential revision: https://reviews.llvm.org/D64805
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llvm-svn: 368017