Const eval incorrectly computes that mutable param is zero

[rinon]

The following code results in a const eval divide by zero error, when it cannot actually error at runtime:

pub fn foo(mut n: i32) {
  if n < 0i32 { n = 0i32 }
  if n > 0i32 {
    dbg!((1i32) / n);
  }
}

https://play.rust-lang.org/?version=nightly&mode=debug&edition=2018&gist=ee4964acbb7fe290cfdaa573fddf5195

[ecstatic-morse]

Bisected to 488381c, which corresponds to #64419. cc @wesleywiser @oli-obk

BTW, you need to actually use foo (e.g. by calling it in main) to trigger the error. Just adding fn main() {} won't do it.

[hellow554]

Fun thing, it doesn't even depend on the conditions in the if clause:

pub fn foo(mut n: i32) {
    if false {
        n = 0i32
    }
    if false {
        dbg!((1i32) / n);
    }
}

also ICEs

[rinon]

Thanks @wesleywiser!