[AMDGPU] [NFC]: Organize the code around reserving registers.

First, add code to reserve all required special purpose registers,
followed by code to reserve SGPRs, followed by code to reserve
VGPRs/AGPRs.

This patch is prepared as a pre-requisite to fix an issue related to
GFX90A hardware.

Reviewed By: arsenm

Differential Revision: https://reviews.llvm.org/D122219