[CodeGenObjC] Use a constant value for non-fragile ivar offsets when possible

If a class inherits from NSObject and has an implementation, then we
can assume that ivar offsets won't need to be updated by the runtime.
This allows us to index into the object using a constant value and
avoid loading from the ivar offset variable.

This patch was adapted from one written by Pete Cooper.

rdar://problem/10132568

Differential revision: https://reviews.llvm.org/D56802

llvm-svn: 351461