Inlining: Run the legacy AlwaysInliner before the regular inliner.

We have several situations where it's beneficial for code size to ensure that every
call to always-inline functions are inlined before normal inlining decisions are
made. While the normal inliner runs in a "MandatoryOnly" mode to try to do this,
it only does it on a per-SCC basis, rather than the whole module. Ensuring that
all mandatory inlinings are done before any heuristic based decisions are made
just makes sense.

Despite being referred to the "legacy" AlwaysInliner pass, it's already necessary
for -O0 because the CGSCC inliner is too expensive in compile time to run at -O0.

This also fixes an exponential compile time blow up in
https://github.com/llvm/llvm-project/issues/59126

Differential Revision: https://reviews.llvm.org/D143624