[AMDGPU] Fix amdgpu_gfx tail-call test

The inreg argument prevented the tail call optimization to kick in.
Remove the inreg, so this test actually uses a tail call.

Note that it now uses s[4:5] for the return address, which is invalid,
because these registers are supposed to be callee-save.
D147096 tried to fix that problem for the C calling convention.

Differential Revision: https://reviews.llvm.org/D148119