cmd/compile/internal/ssa/gen: set wasm genfile

This appears to have been an oversight and/or
left over from development.

Setting the genfile means that extra sanity
checks are executed when regenerating SSA files.
They already pass.

Change-Id: Icc01ecf85020d3d51355e8bccfbc521b52371747
Reviewed-on: https://go-review.googlesource.com/c/154459
Run-TryBot: Josh Bleecher Snyder <josharian@gmail.com>
Reviewed-by: Keith Randall <khr@golang.org>

src/cmd/compile/internal/ssa/gen/WasmOps.go

@@ -196,7 +196,7 @@ func init() {
 	archs = append(archs, arch{
 		name:            "Wasm",
 		pkg:             "cmd/internal/obj/wasm",
-		genfile:         "",
+		genfile:         "../../wasm/ssa.go",
 		ops:             WasmOps,
 		blocks:          nil,
 		regnames:        regNamesWasm,