Rollup merge of #139098 - scottmcm:assert-impossible-tags, r=WaffleLapkin

Tell LLVM about impossible niche tags

I was trying to find a better way of emitting discriminant calculations, but sadly had no luck.

So here's a fairly small PR with the bits that did seem worth bothering:

1. As the [`TagEncoding::Niche` docs](https://doc.rust-lang.org/nightly/nightly-rustc/rustc_abi/enum.TagEncoding.html#variant.Niche) describe, it's possible to end up with a dead value in the input that's not already communicated via the range parameter attribute nor the range load metadata attribute.  So this adds an `llvm.assume` in non-debug mode to tell LLVM about that.  (That way it can tell that the sides of the `select` have disjoint possible values.)

2. I'd written a bunch more tests, or at least made them parameterized, in the process of trying things out, so this checks in those tests to hopefully help future people not trip on the same weird edge cases, like when the tag type is `i8` but yet there's still a variant index and discriminant of `258` which doesn't fit in that tag type because the enum is really weird.

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