[AMDGPU] Modify adjustInliningThreshold to also consider the cost of passing...

[AMDGPU] Modify adjustInliningThreshold to also consider the cost of passing function arguments through the stack

A regression from when new PM got enabled as default. Functions with a big number of instructions will elide getting inlined but do not consider the cost of passing arguments over stack if there are a lot of function arguments. This patch attempts to add a heuristic for AMDGPU's function calling convention that also considers function arguments passed through the stack.

Reviewed By: #amdgpu, arsenm

Differential Revision: https://reviews.llvm.org/D140242