Building with optimizations can skip monomorphizing some types, making errors disappear

This program (original example due to @scottmcm) errors in debug builds but compiles successfully in release builds:

#![crate_type = "lib"]

#[unsafe(no_mangle)]
pub fn f() {
    g::<u16>();
}

pub fn g<T>() -> std::mem::MaybeUninit<[T; 1 << 62]> {
    std::mem::MaybeUninit::uninit()
}

The debug build error is:

error: values of the type `[u16; 4611686018427387904]` are too big for the target architecture
 --> src/lib.rs:8:1
  |
8 | pub fn g<T>() -> std::mem::MaybeUninit<[T; 1 << 62]> {
  | ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^

In a release build, we're optimizing away this function call so the bad type never reaches codegen.

Related issues: #157047, https://github.com/rust-lang/rust/issues/122828.

Reproduce the example in src/lib.rs with both debug and release builds, confirming that the invalid array type is diagnosed only without optimizations. Trace the compiler's monomorphization and code-generation handling for the optimized-away call; done when the invalid type cannot evade the required diagnostic in release builds.