handle equality memcmp of 8 bytes on x86-64 with two unaligned loads and a

handle equality memcmp of 8 bytes on x86-64 with two unaligned loads and a
compare.  On other targets we end up with a call to memcmp because we don't
want 16 individual byte loads.  We should be able to use movups as well, but
we're failing to select the generated icmp.

git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@92107 91177308-0d34-0410-b5e6-96231b3b80d8