R600/SI: SI Control Flow Annotation bug fixed

Mixing of AddAvailableValue and GetValueAtEndOfBlock methods of SSAUpdater
leaded to the endless loop generation when the nested loops annotated.

This fixes a bug in the OCL_ML/KNN OpenCV test.  The test case is too
complex for FileCheck and would be very fragile.

Patch by: Elena Denisova

llvm-svn: 211374