[mips] Stop parsing a .set assignment if the first argument is not an identifier

Before this fix the following code triggers two error messages. The
second one is at least useless:

  test.s:1:9: error: expected identifier after .set
    .set  123, $a0
          ^
  test-set.s:1:9: error: unexpected token, expected comma
    .set  123, $a0
          ^

llvm-svn: 333402