[PrologEpilogInserter][AMDGPU] Only adjust offset for emergency spill slots if the stack grows down

D89239 adjusts the stack offset of emergency spill slots for overaligned
stacks. However the adjustment is not valid for targets whose stack
grows up (such as AMDGPU).

This change makes the adjustment conditional only to those targets whose
stack grows down.

Fixes https://bugs.llvm.org/show_bug.cgi?id=49686

Differential Revision: https://reviews.llvm.org/D99504