[NVPTX] Run NVVMReflect at the beginning of IR passes.

[NVPTX] Run NVVMReflect at the beginning of IR passes.

Summary:
Currently the NVVMReflect pass is run at the beginning of our backend
passes.  But really, it should be run as early as possible, as it's
simply resolving an "if" statement in code.  So copy it into
TargetMachine::addEarlyAsPossiblePasses.

We still run it at the beginning of the backend passes, since it's
needed for correctness when lowering to nvptx.

(Specifically, NVVMReflect changes each call to the __nvvm_reflect
function or llvm.nvvm.reflect intrinsic into an integer constant, based
on the pass's configuration.  Clearly we miss many optimization
opportunities if we perform this transformation at the beginning of
codegen.)

Reviewers: rnk

Subscribers: tra, llvm-commits, jholewinski

Differential Revision: http://reviews.llvm.org/D18616

llvm-svn: 267765