[AMDGPU] Always reserve flat scratch SGPR for architected flat scratch

With architected flat scratch it becomes readonly. We must always
reserve SGPR pair for it even if we do not use scratch at all since
an attempt to write to SGPRs mapped to FLAT_SCRATCH results in
memory violation.

This is not needed since GFX10 with architected flat scratch though
since special SGPRs are not carving space from normal SGPRs.

Differential Revision: https://reviews.llvm.org/D110376