[AMDGPU] Don't fix the scavenge slot at offset 0 (#79136)

At the moment, the emergency spill slot is a fixed object for entry
functions and chain functions, and a regular stack object otherwise.
This patch adopts the latter behaviour for entry/chain functions too. It
seems this was always the intention [1] and it will also save us a bit
of stack space in cases where the first stack object has a large
alignment.

[1]
https://github.com/llvm/llvm-project/commit/34c8b835b16fb3879f1b9770e91df21883356bb6