sched: Do not move expensive insns speculatively (PR68664)

sched: Do not move expensive insns speculatively (PR68664)

Scheduling should never move very expensive instructions to places they
are executed more frequently.  This patch fixes that, reducing the
execution time of c-ray by over 40% (I tested on a BE Power7 system).

This introduces a new target hook sched.can_speculate_insn which returns
whether the scheduler is allowed to speculate a given instruction.  The
rs6000 implementation disallows all divide and square root instructions.

	PR rtl-optimization/68664
	* target.def (can_speculate_insn): New hook.
	* doc/tm.texi.in (TARGET_SCHED_CAN_SPECULATE_INSN): New hook.
	* doc/tm.texi: Regenerate.
	* sched-rgn.c (can_schedule_ready_p): Use the new hook.
	* config/rs6000/rs6000.c (TARGET_SCHED_CAN_SPECULATE_INSN): New macro.
	(rs6000_sched_can_speculate_insn): New function.

From-SVN: r245215