Refactoring codegen passes that lower write barrier and allocation

[udesou]

Moving the lowering functions that are GC-specific (allocation and write barriers) into the final lowering pass.

[udesou]

@vchuravy This draft is an attempt to separate the lowering functions for write barriers and allocation as we discussed on slack. It's still a draft because I'd appreciate your feedback on whether this is the right way to do it. I think one of the tests in llvmpasses is failing so I still need to look into it.
I might also do minor changes to the code but if you think I should do something differently, please let me know.
I have also included the lowering code for MMTk's barrier, for your reference - but that will only be fully working once we support sticky Immix (#57327).

[vchuravy]

(I am on vacation until April, please ping me then)