[AMDGPU] Track physical VGPRs used for SGPR spills (#75573)

Physical VGPRs used for SGPR spills need to be tracked independent of
WWM reserved registers. The WWM reserved set contains extra registers
allocated during WWM pre-allocation pass.

This causes SGPR spills allocated after WWM pre-allocation to overlap
with WWM register usage, e.g. if frame pointer is spilt during
prologue/epilog insertion.