5b7d56f6f7
This PR adds support for `if-let` clauses, as proposed in bytecodealliance/rfcs#21. These clauses augment the left-hand side (pattern-matching phase) of rules in the ISLE instruction-selection DSL with sub-patterns matching on sub-expressions. The ability to list additional match operations to perform, out-of-line from the original toplevel pattern, greatly simplifies some idioms. See the RFC for more details and examples of use.