Refactoring gc_relocate related code in CodeGenPrepare.cpp

Refactoring gc_relocate related code in CodeGenPrepare.cpp

Summary:
The original code inserted new instructions by following a
Create->Remove->ReInsert flow. This patch removes the unnecessary
Remove->ReInsert part by setting up the InsertPoint correctly at the
very beginning. This change does not introduce any functionality change.

Patch by Chen Li!

Reviewers: reames, AndyAyers, sanjoy

Reviewed By: sanjoy

Subscribers: llvm-commits

Differential Revision: http://reviews.llvm.org/D9687

git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@237070 91177308-0d34-0410-b5e6-96231b3b80d8