tcg-ia64: Use SUB_A3 and ADDS_A4 for subtraction

We can subtract from more small constants that just 0 with one insn,
and we can add the negative for most small constants.

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