[AMDGPU] ISel for amdgpu_cs_chain[_preserve] functions

Lower formal arguments and returns for functions with the
`amdgpu_cs_chain` and `amdgpu_cs_chain_preserve` calling conventions:

* Put `inreg` arguments into SGPRs, starting at s0, and other arguments
into VGPRs, starting at v8. No arguments should end up on the stack, if
we don't have enough registers we should error out.

* Lower the return (which is always void) as an S_ENDPGM.

* Set the ScratchRSrc register to s48:51, as described in the docs.

* Set the SP to s32, matching amdgpu_gfx. This might be revisited in a
future patch.

Differential Revision: https://reviews.llvm.org/D153517