AMDGPU/SI: Annotate Loops with Constant Condition in SIAnnotateControlFlow pass.

Summary:
  It is possible that the loop condition can be a boolean constant (infinite loop,
for example). So we sould handle constant condition in annotating a loop. This
patch adds this functionality to support annotating constant condition.

Reviewers: tstellarAMD, arsenm

Subscribers: llvm-commits, arsenm

Differential Revision: http://reviews.llvm.org/D15093

llvm-svn: 260692