The inference is trivial and leverages the MCOI OperandTypes encoded in
CodeGenInstructions to infer types across patterns in a CombineRule. It's
thus very limited and only supports CodeGenInstructions (but that's the main
use case so it's fine).

We only try to infer untyped operands in apply patterns when they're temp
reg defs, or immediates. Inference always outputs a `TypeOf<$x>` where $x is
a named operand from a match pattern.