1.37.20: Inline many (#1125)

* Improve inlining pass to inline single-use functions that are fairly small, which makes it useful for removing unnecessary global constructors from clang.

 * Add an inlining-optimizing pass that also optimizes where it inlined, as new opportunities arise. enable that it by default in O2+

* In addition, in -O3+ also inline small functions with multiple uses. This helps a lot with things like safe-int-divide functions (where each int divide is replaced by a safe divide that won't trap). Inlining gets rid of around half of the overhead there.