Fix a crash in LSR replaceCongruentIVs.

Indirect branch in the preheader crashes replaceCongruentIVs.
Fixes rdar://12910141.

git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@171653 91177308-0d34-0410-b5e6-96231b3b80d8