Revert "require full validity when determining the discriminant of a value"
This reverts commit 0a2b7d71d96a22126cce57f0dab5890d060f2259, reversing
changes made to 47c1bd1bcc50b25d133f8be3d49825491c1df249.
This caused several unforeseen problems:
- https://github.com/rust-lang/rust/issues/91029
- https://github.com/rust-lang/rust/pull/89764#issuecomment-973588007
So I think it's best to revert for now while we keep discussing the MIR semantics of getting a discriminant.
r? `@oli-obk`