Auto merge of #38989 - arielb1:constant-mir-overflow2, r=eddyb

fix function arguments in constant promotion

we can't create the target block until *after* we promote the arguments - otherwise the arguments will be promoted into the target block. oops.

Fixes #38985.

This is a regression introduced in the beta-nominated #38833, so beta-nominating this one too (sorry @brson).

r? @eddyb

src/test/run-pass/issue-37991.rs

@@ -14,7 +14,14 @@ const fn foo() -> i64 {
     3
 }
 
+const fn bar(x: i64) -> i64 {
+    x*2
+}
+
 fn main() {
     let val = &(foo() % 2);
     assert_eq!(*val, 1);
+
+    let val2 = &(bar(1+1) % 3);
+    assert_eq!(*val2, 1);
 }