Remove null checks when operand is non-nullable

[biggs0125]

Follow up to: #59840

One of TFA's tree-shaker passes is normally able to remove null checks that are performed on non-nullable types. However, when assertions are enabled it is possible that the type of variable will get refined to a non-nullable type but a null checks on that local will not get cleared.

Example from other bug:

void main() {
  repro(something: 1.0, other: 1.0);
}

void repro({double? something, double? other}) {
  assert(
    (something == null && other == null) ||
        (something != null && other != null),
  );

  if (something != null) {
    print(something);
    print(other!); // Crash!
  }
}

The type of repro#other is correctly refined to double (not nullable) since the only call site provides a non-null value. However, the assertion in the body of repro causes the non-null check, other!, to be retained. So we end up performing a non-null check on a value that has been inferred to be non-nullable. TFA should be able to check the inferred type and remove this check.

[alexmarkov]

Kernel AST after TFA for this repro (in AOT):

  static method main() → void {
    foo4::repro();
  }
  static method repro() → void {
    {
      core::print(#C1);
      core::print(_in::unsafeCast<core::double>(#C1));
    }
  }

It looks like the null check was removed (and parameters were replaced with constants and removed too).

[biggs0125]

It looks like that was with assertions disabled. The extra check only shows up when assertions are enabled.

[alexmarkov]

Repro which doesn't rely on the assertions:

void main() {
  repro(something: 1.0, other: 1.0);
}

void repro({double? something, double? other}) {
  print(
    (something == null && other == null) ||
        (something != null && other != null)
  );

  if (something != null) {
    print(something);
    print(other!); // Crash!
  }
}

Kernel after TFA:

  static method main() → void {
    foo4::repro();
  }
  static method repro() → void {
    core::print(!(#C1 == null) && true);
    if(!(#C1 == null)) {
      core::print(#C1);
      core::print(#C1!);
    }
  }