compiler: lower func values to switch + direct call

This has several advantages, among them:
  - Many passes (heap-to-stack, dead arg elimination, inlining) do not
    work with function pointer calls. Making them normal function calls
    improves their effectiveness.
  - Goroutine lowering to LLVM coroutines does not currently support
    function pointers. By eliminating function pointers, coroutine
    lowering gets support for them for free.
    This is especially useful for WebAssembly.
Because of the second point, this work is currently only enabled for the
WebAssembly target.

compiler/interface.go

@@ -396,14 +396,10 @@ func (c *Compiler) getInvokeCall(frame *Frame, instr *ssa.CallCommon) (llvm.Valu
 		return llvm.Value{}, nil, err
 	}
 
-	llvmFnType, err := c.getLLVMType(instr.Method.Type())
+	llvmFnType, err := c.getRawFuncType(instr.Method.Type().(*types.Signature))
 	if err != nil {
 		return llvm.Value{}, nil, err
 	}
-	// getLLVMType() has created a closure type for us, but we don't actually
-	// want a closure type as an interface call can never be a closure call. So
-	// extract the function pointer type from the closure.
-	llvmFnType = llvmFnType.Subtypes()[1]
 
 	typecode := c.builder.CreateExtractValue(itf, 0, "invoke.typecode")
 	values := []llvm.Value{