Commits on Mar 15, 2024

1. [Draft][spec] Improve typing of br_if …

   `br_if` previously had type `[t* i32] -> [t*]` where `[t*]` was the type of its
   target label. This typing unnecessarily lost type information in cases where the
   actual input result type is a strict subtype of the label result type,
   requiring casts to recover the types that the validation algorithm already knew
   about before the `br_if`.
   
   Update the type of `br_if` to be `[t1* i32] -> [t1*]` where `[t*]` is a subtype
   of the label type `[t*]`. This type preserves types that were present before the
   `br_if`, even when they are strict subsets of the label result types.
   
   Update the description of principle types in the appendix to allow type
   variables to be constrained by upper bounds and update the validation algorithm
   to reflect the new typing.
   
   This current PR illustrates the intended changes only for `br_if`, but the full
   intended change would similarly improve the types of `br_on_null`,
   `br_on_non_null`, `br_on_cast`, `br_on_cast_fail`, and `local.tee`.

   

   tlively committed Mar 15, 2024
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