Support if conversion for switches

The gimple-if-to-switch pass converts if statements with
multiple equal checks on the same value to a switch. This breaks
vectorization which cannot handle switches.

Teach the tree-if-conv pass used by the vectorizer to handle
simple switch statements, like those created by if-to-switch earlier.
These are switches that only have a single non default block,
They are handled similar to COND in if conversion.

This makes the vect-bitfield-read-1-not test fail. The test
checks for a bitfield analysis failing, but it actually
relied on the ifcvt erroring out early because the test
is using a switch. The if conversion still does not
work because the switch is not in a form that this
patch can handle, but it fails much later and the bitfield
analysis succeeds, which makes the test fail. I marked
it xfail because it doesn't seem to be testing what it wants
to test.

	PR tree-optimization/115866

gcc/ChangeLog:

	* tree-if-conv.cc (if_convertible_switch_p): New function.
	(if_convertible_stmt_p): Check for switch.
	(get_loop_body_in_if_conv_order): Handle switch.
	(predicate_bbs): Likewise.
	(predicate_statements): Likewise.
	(remove_conditions_and_labels): Likewise.
	(ifcvt_split_critical_edges): Likewise.
	(ifcvt_local_dce): Likewise.

gcc/testsuite/ChangeLog:

	* gcc.dg/vect/vect-switch-ifcvt-1.c: New test.
	* gcc.dg/vect/vect-switch-ifcvt-2.c: New test.
	* gcc.dg/vect/vect-switch-search-line-fast.c: New test.
	* gcc.dg/vect/vect-bitfield-read-1-not.c: Change to xfail.