Limit outer-loop unswitching by duplicated code size

Limit outer-loop unswitching by duplicated code size

When unswitching predicates from the innermost loop, hoisting the
unswitch out to an outer loop duplicates only the outer-loop bodies;
the innermost loop is shared.  Estimate the cost as
candidate_size - innermost_size and stop selecting an outer loop once
that exceeds param_max_unswitch_insns.  innermost_size is hoisted out
of the walk so estimate_loop_insns is called once per level.

gcc/ChangeLog:

	* tree-ssa-loop-unswitch.cc (estimate_loop_insns): New function.
	(init_loop_unswitch_info): Do not select an outer loop for
	unswitching when the duplicated outer-body size exceeds
	param_max_unswitch_insns.

gcc/testsuite/ChangeLog:

	* gcc.dg/loop-unswitch-19.c: New test.