Fix crasher introduced in r200203 and caught by a libc++ buildbot. Don't assume that getMulExpr returns a SCEVMulExpr, it may have simplified it to something else!


git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@200210 91177308-0d34-0410-b5e6-96231b3b80d8