Some misc compiler-related code cleanup (JuliaLang#56540)

I'm cleaning up some downstream packages following the compiler split
and these were commonly found in compat code. One is the `Base.copy`
method for Phi(C)Node, which we had an implementation for, just not
wired up to `copy`. The other is `block_for_inst` with an SSAValue,
which exists for IncremetalCompact, but not IRCode, so add the latter
for consistency.

Author: Keno

src/ssair/ir.jl

@@ -482,6 +482,7 @@ function block_for_inst(ir::IRCode, inst::Int)
     end
     block_for_inst(ir.cfg, inst)
 end
+block_for_inst(ir::IRCode, ssa::SSAValue) = block_for_inst(ir, ssa.id)
 
 function getindex(ir::IRCode, s::SSAValue)
     id = s.id