[msan] Use explicitely aligned stores and loads with function argument shadow.

[msan] Use explicitely aligned stores and loads with function argument shadow.

Use explicitely aligned store and load instructions to deal with argument and
retval shadow. This matters when an argument's alignment is higher than
__msan_param_tls alignment (which is the case with __m128i).

git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@169859 91177308-0d34-0410-b5e6-96231b3b80d8