Don't require src/dst patterns to be able to fully resolve their types,

because information about one can help refine the other.  This allows us to
write:

def : Pat<(i32 (extload xaddr:$src, i8)),
          (LBZX xaddr:$src)>;

as:

def : Pat<(extload xaddr:$src, i8),
          (LBZX xaddr:$src)>;

because tblgen knows LBZX returns i32.

llvm-svn: 28865