[InstCombine] Use getDestAlignment in SimplifyMemSet (NFC)

Summary:
Small NFC change to change the name of the function used getting and setting
the alignment of a memset.

git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@324148 91177308-0d34-0410-b5e6-96231b3b80d8
1 file changed