Switch to the new LLVM pass manager

[kripken]

LLVM has made the switch, and are releasing it to the general public
any day now in the new release. It doesn't really make sense for us to
be more careful about things than them.

Fixes #13427. As noted there, this does regress some things like the
minor code size increase in the test updates here, but also it helps on
some benchmarks. The overall tradeoff is good in LLVM's eyes.

For the LLVM compile time regression, I looked at how #13420
tried to fix it, but it's actually quite hard - almost every single macro
use there needs to be put in its own function, to avoid really bad
compile times. -O3 is just very very aggressive on those loops it
seems. Instead of a big refactoring, I disabled those tests on -O3
specifically. -O2 is close enough to give us good coverage there
I think - the main difference between them is inlining, after all.