Codegen emits a specsig call in multiple places which bitrots easily

[gbaraldi]

There is a couple TODOs in the codebase for it, and @topolarity asked me to open this issue.
Example

julia/src/codegen.cpp

Line 7204 in 0654f8d

AllocaInst *return_roots = emit_static_alloca(ctx, get_returnroots_type(ctx, returninfo.return_roots));