[msan] Raise alignment of origin stores/loads when possible.

[msan] Raise alignment of origin stores/loads when possible.

Origin alignment is as high as the alignment of the corresponding application
location, but never less than 4.

git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@171110 91177308-0d34-0410-b5e6-96231b3b80d8