Since multivalue was standardized, WebAssembly has supported not only multiple results but also an arbitrary number of inputs on control flow structures, but until now Binaryen did not support control flow input. Binaryen IR still has no way to represent control flow input, so lower it away using scratch locals in IRBuilder. Since both the text and binary parsers use IRBuilder, this gives us full support for parsing control flow inputs. The lowering scheme is mostly simple. A local.set writing the control flow inputs to a scratch local is inserted immediately before the control flow structure begins and a local.get retrieving those inputs is inserted inside the control flow structure before the rest of its body. The only complications come from ifs, in which the inputs must be retrieved at the beginning of both arms, and from loops, where branches to the beginning of the loop must be transformed so their values are written to the scratch local along the way. Resolves #6407.