The NonZero types don't tell LLVM that they're non-zero on get

Repro: https://play.rust-lang.org/?gist=0981601dda9c5800e353e31b22682bb5&version=nightly&mode=release
```rust
pub fn foo(x: std::num::NonZeroU32) -> bool {
    x.get() != 0
}
```
Actual:
```llvm
  %0 = icmp ne i32 %x, 0
  ret i1 %0
```
Expected:
```llvm
  ret i1 true
```
cc https://github.com/rust-lang/rust/issues/49137