[AMDGPU] move hasAGPRs/hasVGPRs into header

It is now very simple and can go right into the header
allowing optimizer to combine callers, such as isVGPRClass
and similar.

It does not need anything from the TRI itself anymore, so
make it static class member along with the callers.

Differential Revision: https://reviews.llvm.org/D110762