[AMDGPU][IndirectCalls] Fix register usage propagation for indirect/external calls

This patch computes max SGPRs and VGPRs used by module
in presence of indirect calls and makes that
as register requirement for functions/kernels
which makes indirect calls.

This patch also refactors code AMDGPUSubTarget.cpp
which add a "base" variants of getMaxNumSGPRs which
is used by MachineFunction and new Function version.

Reviewed By: arsenm

Differential Revision: https://reviews.llvm.org/D103636