[AMDGPU] Simpler names for arch-specific ttmp registers. NFC.

Rename the *_gfx9_gfx10 ttmp registers to *_gfx9plus for simplicity,
and use the corresponding isGFX9Plus predicate to decide when to use
them instead of the old *_vi versions.

Differential Revision: https://reviews.llvm.org/D94975