[AMDGPU] SILowerSGPRSpills: do not update MRI reserve registers (#77888)

VGPRs used for spilling do not require explicit reservation with MRI.
freezeReservedRegs() executed before register allocation ensures these
are placed in the reserve set.

The only pass after SILowerSGPRSpills is SIPreAllocateWWMRegs which
explicitly tests for interference before register allocation so should
not reuse a WWM VGPR holding spill data. reserveReg prevents calculation
of correct liveness for physical registers which could be used to extend
SIPreAllocateWWMRegs.