[msan] Fix vector pack intrinsic handling.

This fixes a crash on MMX intrinsics, as well as a corner case in handling of
all unsigned pack intrinsics.

PR19953.


git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@210454 91177308-0d34-0410-b5e6-96231b3b80d8