ifcvt: Support bitfield lowering of multiple-exit loops

With the patch enabling the vectorization of early-breaks, we'd like to allow
bitfield lowering in such loops, which requires the relaxation of allowing
multiple exits when doing so.  In order to avoid a similar issue to PR107275,
the code that rejects loops with certain types of gimple_stmts was hoisted from
'if_convertible_loop_p_1' to 'get_loop_body_in_if_conv_order', to avoid trying
to lower bitfields in loops we are not going to vectorize anyway.

This also ensures 'ifcvt_local_dec' doesn't accidentally remove statements it
shouldn't as it will never come across them.  I made sure to add a comment to
make clear that there is a direct connection between the two and if we were to
enable vectorization of any other gimple statement we should make sure both
handle it.

gcc/ChangeLog:

	* tree-if-conv.cc (if_convertible_loop_p_1): Move check from here ...
	(get_loop_body_if_conv_order): ... to here.
	(if_convertible_loop_p): Remove single_exit check.
	(tree_if_conversion): Move single_exit check to if-conversion part and
	support multiple exits.

gcc/testsuite/ChangeLog:

	* gcc.dg/vect/vect-bitfield-read-1-not.c: New test.
	* gcc.dg/vect/vect-bitfield-read-2-not.c: New test.
	* gcc.dg/vect/vect-bitfield-read-8.c: New test.
	* gcc.dg/vect/vect-bitfield-read-9.c: New test.

Co-Authored-By:  Andre Vieira <andre.simoesdiasvieira@arm.com>

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