Enable fan-in heuristics at high opt, reduce method size threshold from 50 to 30 for all opt levels

[ymanton]

The commits in this PR enable the JIT compiler's fan-in heuristics at >warm opt levels and reduce the method size threshold from 50 bytecodes to 30. Methods that are smaller than the threshold are not considered by the fan-in heuristic. By reducing the threshold we allow more methods to be considered by the heuristic, which tries to prevent inlining of methods that are called from a large number of call sites (i.e. that have a high fan-in). This change shows positive effects on certain benchmarks that have small methods that are called from thousands of call sites.

The patches also add the TR_disableFanInAtHighOpt env var which can be set to prevent the fan-in heuristic from being used at >warm.

(The method size threshold can already be controlled via the -Xjit:iprofilerFaninMethodMinSize= option.)

[ymanton]

FYI @vijaysun-omr.

I've opened this as a draft for the time being so that I can evaluate some other alternatives. Namely whether enabling fan-in at >warm is worthwhile, whether leaving the threshold size at 50 and instead removing the threshold check from one of the two places it's used in is beneficial, and so on.