cfgcleanup: Fold jumps and conditional branches with returns

cfgcleanup: Fold jumps and conditional branches with returns

This patch makes cfgcleanup optimize jumps to returns.  There are three
cases this handles:

-- A jump to a return; this is simplified to just that return.
-- A conditional branch to a return; simplified to a conditional return.
-- A conditional branch that falls through to a return.  This is simplified
   to a conditional return (with the condition inverted), falling through
   to a jump to the original destination.  That jump can then be optimized
   further, as usual.

This handles all cases the current function.c does, and a few it misses.

	* cfgcleanup.c (bb_is_just_return): New function.
	(try_optimize_cfg): Simplify jumps to return, branches to return,
	and branches around return.

From-SVN: r235904