There is an issue on ARM where a function wont be inlined due to
mismatching target features between caller and callee.
The caller has `HasV8Ops` and `FeatureDotProd` and the callee does not,
but AFAIK this should not be a problem.
https://godbolt.org/z/f19h3zT66 is an example showing how the call is
not inlined on armv7.
The expected asm output would be something like:
```asm
.fnstart
vsdot.s8 q0, q1, d4[0]
bx lr
.Lfunc_end0:
```
Thanks to @Amichaxx we managed to narrow it down and now can resolve
this problem by adding `ARM::FeatureDotProd, ARM::HasV8Ops` to
InlineFeaturesAllowed in llvm/lib/Target/ARM/ARMTargetTransformInfo.h,
after which the inlining occurs successfully.
Whilst we're at it we have also added some debugging to make it easier
to tell why (or why not) a function is being inlined for ARM, and a
couple other features that seem to be missing from the list.
This patch was motivated by an issue experienced with rust that was
traced back to llvm, and thus was designed to address that.
fab06fae00