[AMDGPU] Skip stack-arg dbg objects while fixing the dead frame indices

Both SGPR->VGPR and VGPR->AGPR spilling code give a fixup to the
spill frame indices referred in debug instructions so that they
can be entirely removed. We should skip the stack argument debug
objects while looking inside the bitvector with FI as the index
that tracks the spill indices being processed. The stack args will
have negative indices and would crash while accessing the bitvector.

Reviewed By: arsenm

Differential Revision: https://reviews.llvm.org/D137277