[ty] Only normalize constraint bounds for display (#21516)
We were previously normalizing the upper and lower bounds of each
constraint when constructing constraint sets. Like in #21463, this was
for conflated reasons: It made constraint set displays nicer, since we
wouldn't render multiple constraints with obviously equivalent bounds.
(Think `T ≤ A & B` and `T ≤ B & A`) But it was also useful for
correctness, since prior to #21463 we were (trying to) add the full
transitive closure to a constraint set's BDD, and normalization gave a
useful reduction in the number of nodes in a typical BDD.
Now that we don't store the transitive closure explicitly, that second
reason is no longer relevant. Our sequent map can store that full
transitive closure much more efficiently than the expanded BDD would
have. This helps fix some false positives on #20933, where we're seeing
some (incorrect, need to be fixed, but ideally not blocking this effort)
assignability failures between a type and its normalization.
Normalization is still useful for display purposes, and so we do
normalize the upper/lower bounds before building up our display
representation of a constraint set BDD.
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Co-authored-by: David Peter <sharkdp@users.noreply.github.com>
