[AMDGPU] Graceful abort for waterfalls in SIOptimizeVGPRLiveRange

If the CFG structure of a waterfall loop is not the expected shape
then gracefully abort traversing the IR for the given loop.
This applies to nest waterfall loops which are not supported by
the VGPR live range optimizer.

Reviewed By: ruiling

Differential Revision: https://reviews.llvm.org/D123480