Fix stored type of arguments on x86-64

Fix stored type of arguments on x86-64

the lvalue Syms for arguments didn't correctly reflect
their own types in all cases (a side-effect of the type being
stored in type->t and the ->r members (as VT_LVAL_xxx), so the below
used an int load (not a byte load) in the conditional.

extern void bar (void);
void foo (signed char c)
{
  signed char x = c;
  if (c)
    bar();
}