Constexpr evaluation in impl methods doesn't always work

[grebe]

Describe the bug
const_assert!() doesn't seem to be able to constexpr-evaluate function parametrics.

To Reproduce

pub struct A {}
impl A {
  fn B<C:u32>() -> bool {
      const_assert!(C >= u32:4);
      true
  }
}

Expected behavior
It should typecheck and the assertion should pass or fail base on invocation.

Instead, I get TypeInferenceError: const_assert! expression is not constexpr regardless of the invocations of A::B<...>().