Enable FeatureFastUAMem for btver2

Allow unaligned 16-byte memop codegen for btver2. No functional changes for any other subtargets.

Replace the existing supposed small memcpy test with an actual test of a small memcpy.
The previous test wasn't using FileCheck either.

This patch should allow us to close PR21541 ( http://llvm.org/bugs/show_bug.cgi?id=21541 ).

Differential Revision: http://reviews.llvm.org/D6360