[AMDGPU][Scheduler] Avoid initializing Register pressure tracker when tracking is disabled

When register pressure tracking is disabled, the scheduler attempts to load
pressures at SReg_32 and VGPR_32. This causes an index out of bounds error.
This patch fixes this issue by disabling the initialization of RPTracker
when not needed. NFC

Reviewed By: rampitec, kerbowa, arsenm

Differential Revision: https://reviews.llvm.org/D129322