Auto merge of #124999 - scottmcm:unify-aggregate, r=nnethercote

Unify `Rvalue::Aggregate` paths in cg_ssa

In #123840 and #123886 I added two different codepaths for `Rvalue::Aggregate` in `cg_ssa`.

This merges them into one, since raw pointers are also immediates that can be built from the immediates of their "fields".

Author: bors