Desugaring of as-patterns in let bindings are insufficiently strict.

[expipiplus1]

For example:

• original: let a@1 = 2 in a -> pattern match failure
• with dsPatX: let {1 = 2; a_0 = 1} in a_0 -> 1
• with dsPat': let {a_0 = 2; 1 = a_0} in a_0 -> 2

This should actually be something like let a = case 2 of 1 -> 2 in a (which is actually what is generated for the pattern synonym case.