tcg: Split out swap_commutative as a subroutine

tcg: Split out swap_commutative as a subroutine

Reduces code duplication and prefers

  movcond d, c1, c2, const, s
to
  movcond d, c1, c2, s, const

It also prefers

  add r, r, c
over
  add r, c, r

when both inputs are known constants.  This doesn't matter for true add, as
we will fully constant fold that.  But it matters for a follow-on patch using
this routine for add2 which may not be fully foldable.

Signed-off-by: Richard Henderson <rth@twiddle.net>
Signed-off-by: Aurelien Jarno <aurelien@aurel32.net>