[AMDGPU] Add amdgpu_gfx calling convention

[AMDGPU] Add amdgpu_gfx calling convention

Add a calling convention called amdgpu_gfx for real function calls
within graphics shaders. For the moment, this uses the same calling
convention as other calls in amdgpu, with registers excluded for return
address, stack pointer and stack buffer descriptor.

Differential Revision: https://reviews.llvm.org/D88540