[RS4GC] Cosmetic cleanup, NFC

Summary:
A series of cosmetic cleanup changes to RewriteStatepointsForGC:

  - Rename variables to LLVM style
  - Remove some redundant asserts
  - Remove an unsued `Pass *` parameter
  - Remove unnecessary variables
  - Use C++11 idioms where applicable
  - Pass CallSite by value, not reference

Reviewers: reames, swaroop.sridhar

Subscribers: llvm-commits, sanjoy

Differential Revision: http://reviews.llvm.org/D13370

llvm-svn: 249508