[AMDGPU][Inliner] Remove amdgpu-inline and add a new TTI inline hook

Having a custom inliner doesn't really fit in with the new PM's
pipeline. It's also extra technical debt.

amdgpu-inline only does a couple of custom things compared to the normal
inliner:
1) It disables inlining if the number of BBs in a function would exceed
   some limit
2) It increases the threshold if there are pointers to private arrays(?)

These can all be handled as TTI inliner hooks.
There already exists a hook for backends to multiply the inlining
threshold.

This way we can remove the custom amdgpu-inline pass.

This caused inline-hint.ll to fail, and after some investigation, it
looks like getInliningThresholdMultiplier() was previously getting
applied twice in amdgpu-inline (https://reviews.llvm.org/D62707 fixed it
not applying at all, so some later inliner change must have fixed
something), so I had to change the threshold in the test.

Reviewed By: rampitec

Differential Revision: https://reviews.llvm.org/D94153