compiler: simplify runtime.interfaceMethod signature slightly

Code size is not affected, as dead arguments are eliminated anyway.
It makes future interface optimizations hopefully easier to implement.

compiler/compiler.go

@@ -2272,8 +2272,9 @@ func (c *Compiler) getInvokeCall(frame *Frame, instr *ssa.CallCommon) (llvm.Valu
 		llvmFnType = llvmFnType.Subtypes()[1]
 	}
 
+	typecode := c.builder.CreateExtractValue(itf, 0, "invoke.typecode")
 	values := []llvm.Value{
-		itf,
+		typecode,
 		llvm.ConstInt(c.ctx.Int16Type(), uint64(c.ir.MethodNum(instr.Method)), false),
 	}
 	fn := c.createRuntimeCall("interfaceMethod", values, "invoke.func")

src/runtime/interface.go

@@ -48,12 +48,12 @@ var (
 // Get the function pointer for the method on the interface.
 // This is a compiler intrinsic.
 //go:nobounds
-func interfaceMethod(itf _interface, method uint16) *uint8 {
+func interfaceMethod(typecode uint16, method uint16) *uint8 {
 	// This function doesn't do bounds checking as the supplied method must be
 	// in the list of signatures. The compiler will only emit
 	// runtime.interfaceMethod calls when the method actually exists on this
 	// interface (proven by the typechecker).
-	i := methodSetRanges[itf.typecode-firstTypeWithMethods].index
+	i := methodSetRanges[typecode-firstTypeWithMethods].index
 	for {
 		if methodSetSignatures[i] == method {
 			return methodSetFunctions[i]