[AMDGPU] Fix scheduler live-ins with debug inst at start of block

GCNDownwardRPTracker RPTracker.reset() skips debug instructions for NextMI so RPTracker.getNext() will never give the beginning of a sched region if it is a debug value. In this case we will never set the live-ins for that block.

Add check to see if getNext also equals the MI after skipping debug instructions.

Reviewed By: rampitec

Differential Revision: https://reviews.llvm.org/D118853