[AMDGPU] Allow to use a whole register file on gfx90a for VGPRs

In a kernel which does not have calls or AGPR usage we can allocate
the whole vector register budget for VGPRs and have no AGPRs as
long as VGPRs stay addressable (i.e. below 256).

Differential Revision: https://reviews.llvm.org/D111764