[AMDGPU] Fix GCNDownwardRPTracker::advanceBeforeNext at the end of MBB

The problem with GCNDownwardRPTracker::advanceBeforeNext is that it doesn't allow to get register pressure
after the last instruction in a MBB.

However when we track RP through the boundary of a MBB we need the state that is after the last instruction
of the MBB and before the first instruction of the successor MBB. Currently we stop traking RP in the state
 'at' the last instruction of the MBB which is incorrect.

This patch fixes 27 lit tests with EXPENSIVE_CHECKS enabled.

Reviewed By: rampitec, arsenm

Differential Revision: https://reviews.llvm.org/D136927