Reduce code duplication between patchpoint and non-patchpoint lowering. NFC.

This is in preparation for another patch that makes patchpoints invokable.

Reviewers: atrick, ributzka
Reviewed By: ributzka
Subscribers: llvm-commits

Differential Revision: http://reviews.llvm.org/D5657

git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@219967 91177308-0d34-0410-b5e6-96231b3b80d8