Nondeterminism in GC optimizations

[tlively]

One of these passes definitely has nondeterminism:

• --intrinsic-lowering
• --remove-unused-module-elements

Additionally, there is more determinism somewhere in optimization pipelines with these flags:

• --partial-inlining-ifs=4 -fimfs=50 --gufa --intrinsic-lowering -O3

[kripken]

This is helped by #5590 but it is unclear if that is the only source of nondeterminism.