Restore setindex!(::IRCode, ::SSAValue, ::SSAValue) (#45216)

This particular method got removed in f67371d, but it's common
in RAUW-like patterns in external code. I think it is a sensible
method to have, because the metadata on such an instruction will
get deleted on the next compact anyway, so there's not much reason
to attempt to force the API user to pass complete metadata around.
It also enables useful patterns like:
```
ir[idx] = insert_node!(idx-2, NewInstruction(...))
```

base/compiler/ssair/ir.jl

@@ -151,6 +151,20 @@ function first_insert_for_bb(code, cfg::CFG, block::Int)
     error("any insert position isn't found")
 end
 
+# SSA values that need renaming
+struct OldSSAValue
+    id::Int
+end
+
+# SSA values that are in `new_new_nodes` of an `IncrementalCompact` and are to
+# be actually inserted next time (they become `new_nodes` next time)
+struct NewSSAValue
+    id::Int
+end
+
+const AnySSAValue = Union{SSAValue, OldSSAValue, NewSSAValue}
+
+
 # SSA-indexed nodes
 
 struct NewInstruction
@@ -253,6 +267,10 @@ function setindex!(is::InstructionStream, newval::Instruction, idx::Int)
     is.flag[idx] = newval[:flag]
     return is
 end
+function setindex!(is::InstructionStream, newval::AnySSAValue, idx::Int)
+    is.inst[idx] = newval
+    return is
+end
 function setindex!(node::Instruction, newval::Instruction)
     node.data[node.idx] = newval
     return node
@@ -312,7 +330,7 @@ function getindex(x::IRCode, s::SSAValue)
     end
 end
 
-function setindex!(x::IRCode, repl::Instruction, s::SSAValue)
+function setindex!(x::IRCode, repl::Union{Instruction, AnySSAValue}, s::SSAValue)
     if s.id <= length(x.stmts)
         x.stmts[s.id] = repl
     else
@@ -321,19 +339,6 @@ function setindex!(x::IRCode, repl::Instruction, s::SSAValue)
     return x
 end
 
-# SSA values that need renaming
-struct OldSSAValue
-    id::Int
-end
-
-# SSA values that are in `new_new_nodes` of an `IncrementalCompact` and are to
-# be actually inserted next time (they become `new_nodes` next time)
-struct NewSSAValue
-    id::Int
-end
-
-const AnySSAValue = Union{SSAValue, OldSSAValue, NewSSAValue}
-
 mutable struct UseRefIterator
     stmt::Any
     relevant::Bool