Fix invalid load generated by gfunc_return()

On backends that rely on gfunc_return() to handle structures
  returned in registers (like RISC-V), gfunc_return() may generate
  invalid loads for structures without VT_LOCAL and VT_LVAL. This
  commit fixes it and adds a regression test
  (131_return_struct_in_reg)

tests/tests2/131_return_struct_in_reg.c

@@ -0,0 +1,49 @@
+#include<stdio.h>
+
+#define DATA 0x1234567890abcd, 0x5a5aa5a5f0f00f0f
+
+struct s {
+	long int a;
+	long int b;
+};
+
+struct {
+	struct s d;
+} g = { { DATA } }, *gp = &g;
+
+struct s
+foo1(void)
+{
+	struct s d = { DATA };
+	struct s *p = &d;
+	long int x = 0;
+	return *p;
+}
+
+struct s
+foo2(void)
+{
+	long int unused = 0;
+	return gp->d;
+}
+
+struct s
+foo3(void)
+{
+	struct s d = { DATA };
+	long int unused = 0;
+	return d;
+}
+
+int
+main(void)
+{
+	struct s d;
+	d = foo1();
+	printf("%lx %lx\n", d.a, d.b);
+	d = foo2();
+	printf("%lx %lx\n", d.a, d.b);
+	d = foo3();
+	printf("%lx %lx\n", d.a, d.b);
+	return 0;
+}

tests/tests2/131_return_struct_in_reg.expect

@@ -0,0 +1,3 @@
+1234567890abcd 5a5aa5a5f0f00f0f
+1234567890abcd 5a5aa5a5f0f00f0f
+1234567890abcd 5a5aa5a5f0f00f0f