[IRShow] expose index information to postprinter (#41828)

I am experimenting with showing some other information besides just the
types here as well and I think this would be generally useful. This
passes that information as an `IOContext` as to not break any downstream
uses of this code.

(cherry picked from commit c2b4b38)

base/compiler/ssair/show.jl

@@ -628,7 +628,7 @@ function show_ir_stmt(io::IO, code::Union{IRCode, CodeInfo}, idx::Int, line_info
         if new_node_type === UNDEF # try to be robust against errors
             printstyled(io, "::#UNDEF", color=:red)
         elseif show_type
-            line_info_postprinter(io, new_node_type, node_idx in used)
+            line_info_postprinter(IOContext(io, :idx => node_idx), new_node_type, node_idx in used)
         end
         println(io)
         i += 1
@@ -643,7 +643,7 @@ function show_ir_stmt(io::IO, code::Union{IRCode, CodeInfo}, idx::Int, line_info
             # This is an error, but can happen if passes don't update their type information
             printstyled(io, "::#UNDEF", color=:red)
         elseif show_type
-            line_info_postprinter(io, type, idx in used)
+            line_info_postprinter(IOContext(io, :idx => idx), type, idx in used)
         end
     end
     println(io)