Ignore less tests in debug builds

Since rust-lang/rust#120863 and rust-lang/rust#120863, nearly all UB-detecting debug assertions get compiled out of code that is monomorphized by a crate built with debug assertions disabled.

Which means that if we default all our codegen tests to `-Cdebug-assertions=no`, most of them work just fine against a sysroot built with debug assertions.

I also tried to explain a bit better why some tests need to be skipped, for those that still need to be skipped.