(all functions): Use NULL_RTX instead of 0 for rtx comparisons.

(all functions): Use NULL_RTX instead of 0 for rtx comparisons.

(emit_pop_insn): Emit insn with mode QImode: insn contains stack-regs.
(emit_hard_swap_insn): Delete.
(emit_swap_insn): Merge with emit_hard_swap_insn.
Delete WHEN arg and change all callers.
If previous insn was a push from REG, omit swap.
If previous insn wrote reg-stack top to REG, omit swap.
Emit swap insn with mode QImode: insn contains stack-regs.
Emit swap insn after previous float insn, instead of before this one.
(swap_rtx_condition): New function.
(compare_for_stack_reg): If op 0 of COMPARE is not a stack reg, exchange
ops 0 and 1 and swap cc0 user condition.

From-SVN: r3294