Improve forward jump threading of switch statements (PR18046)

Improve forward jump threading of switch statements (PR18046)

gcc/ChangeLog:

	PR tree-optimization/18046
	* tree-ssa-threadedge.c: Include cfganal.h.
	(simplify_control_statement_condition): If simplifying a
	GIMPLE_SWITCH, replace the index operand of the GIMPLE_SWITCH
	with the dominating ASSERT_EXPR before handing it off to VRP.
	Mention that a CASE_LABEL_EXPR may be returned.
	(thread_around_empty_blocks): Adjust to handle
	simplify_control_statement_condition() returning a
	CASE_LABEL_EXPR.
	(thread_through_normal_block): Likewise.
	* tree-vrp.c (simplify_stmt_for_jump_threading): Simplify
	a switch statement by trying to determine which case label
	will be taken.

gcc/testsuite/ChangeLog:

	PR tree-optimization/18046
	* gcc.dg/tree-ssa/vrp105.c: New test.
	* gcc.dg/tree-ssa/vrp106.c: New test.

From-SVN: r239181