Fix invalidations in finish_show_ir (#48134)

The whole module is under `@nospecialize`, so inference
needs us to annotate the argtypes.

Author: timholy

base/compiler/ssair/show.jl

@@ -858,7 +858,7 @@ function show_ir_stmts(io::IO, ir::Union{IRCode, CodeInfo, IncrementalCompact},
     return bb_idx
 end
 
-function finish_show_ir(io::IO, cfg, config::IRShowConfig)
+function finish_show_ir(io::IO, cfg::CFG, config::IRShowConfig)
     max_bb_idx_size = length(string(length(cfg.blocks)))
     config.line_info_preprinter(io, " "^(max_bb_idx_size + 2), 0)
     return nothing