(subsi3): Don't bother calling negate_rtx for constant; we shouldn't be
called with a constant.
(maskir patterns): Don't mention an operand before it is used; use operand
number 1 for operand with '0' constraint.
(allocate_stack): Use reg_or_short_operand for predicate of allocation amount;
use negate_rtx and always make a PLUS.

From-SVN: r2262