Auto merge of #62584 - eddyb:circular-math-is-hard, r=pnkfelix

 rustc_codegen_ssa: fix range check in codegen_get_discr.

Fixes #61696, see https://github.com/rust-lang/rust/issues/61696#issuecomment-505473018 for more details.

In short, I had wanted to use `x - a <= b - a` to check whether `x` is in `a..=b` (as it's 1 comparison instead of 2 *and* `b - a` is guaranteed to fit in the same data type, while `b` itself might not), but I ended up with `x - a + c <= b - a + c` instead, because `x - a + c` was the final value needed.

That latter comparison is equivalent to checking that `x` is in `(a - c)..=b`, i.e. it also includes `(a - c)..a`, not just `a..=b`, so if `c` is not `0`, it will cause false positives.

This presented itself as the non-niche ("dataful") variant sometimes being treated like a niche variant, in the presence of uninhabited variants (which made `c`, aka the index of the first niche variant, arbitrarily large).

r? @nagisa, @rkruppe or @oli-obk

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