fix: erase code of an erased type in LCNF simp (#8712)

This PR optimizes let decls of an erased type to an erased value.
Specialization can create local functions that produce a Prop, and
there's no point in keeping them around.

Author: zwarich

src/Lean/Compiler/LCNF/Simp/Main.lean

@@ -224,6 +224,9 @@ partial def simp (code : Code) : SimpM Code := withIncRecDepth do
     let mut decl ← normLetDecl baseDecl
     if baseDecl != decl then
       markSimplified
+    if decl.type.isErased && decl.value != .erased then
+      markSimplified
+      decl ← decl.updateValue .erased
     if let some value ← simpValue? decl.value then
       markSimplified
       decl ← decl.updateValue value