[AMDGPU][PEI] Set up SP for chain functions

Initialize the SP to 0 in the prologue of functions with the
`amdgpu_cs_chain` or `amdgpu_cs_chain_preserve` calling conventions, but
only if they need one (i.e. if they contain calls to `amdgpu_gfx`
functions or if they have stack objects).

Also make sure we don't try to realign the stack (since 0 is aligned
enough).

Differential Revision: https://reviews.llvm.org/D156413