cranelift: simplify x-x to 0 (#6032)

* cranelift: simplify `x-x` to `0`

* Guard `x-x => 0` rewrite with `fits_in_64`

cranelift/codegen/src/opts/algebraic.isle

@@ -35,6 +35,9 @@
 ;; ineg(ineg(x)) == x.
 (rule (simplify (ineg ty (ineg ty x))) (subsume x))
 
+;; x-x == 0.
+(rule (simplify (isub (fits_in_64 (ty_int ty)) x x)) (subsume (iconst ty (imm64 0))))
+
 ;; x*1 == 1*x == x.
 (rule (simplify (imul ty
                       x

cranelift/filetests/filetests/egraph/algebraic.clif

@@ -232,6 +232,14 @@ block0(v0: i32):
     ; check: return v0
 }
 
+function %isub_self(i32) -> i32 {
+block0(v0: i32):
+    v1 = isub v0, v0
+    return v1
+    ; check: v2 = iconst.i32 0
+    ; check: return v2
+}
+
 function %or_and_y_with_not_y_i8(i8, i8) -> i8 {
 block0(v0: i8, v1: i8):
     v2 = band v0, v1