When we promote a load of an argument make sure to take the alignment

When we promote a load of an argument make sure to take the alignment
of the previous load - it's usually important.  For example, we don't want
to blindly turn an unaligned load into an aligned one.

git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@99699 91177308-0d34-0410-b5e6-96231b3b80d8