[AMDGPU] Scheduler: fix RP calculation for a MBB with one successor

We reuse live registers after tracking one MBB as live-ins to the successor MBB
if the successor is only one but we don't check if the successor has other predecessors.

`A   B`
` \ /`
`  C`

A and B have one successor but C has live-ins defined by A and B and therefore should be
initialized using LIS.

This fixes 83 lit tests out if 420 with EXPENSIVE_CHECK enabled.

Reviewed By: rampitec

Differential Revision: https://reviews.llvm.org/D136918