Fix const prop ICE

we used to erase the local just before we tried to read it for diagnostics

src/test/ui/const_prop/ice-assert-fail-div-by-zero.rs

@@ -0,0 +1,9 @@
+// check-pass
+
+pub struct Fixed64(i64);
+
+pub fn div(f: Fixed64) {
+    f.0 / 0;
+}
+
+fn main() {}