[AMDGPU] For PAL, make sure Scratch Buffer Descriptor do not clobber GIT pointer
Since SRSRC has alignment requirements, first find non GIT pointer clobbered registers for SRSRC and then if those registers clobber preloaded Scratch Wave Offset register, copy the Scratch Wave Offset register to a free SGPR.
Hoist getGITPtrLoReg out of the loop