reassoc vs uninitialized variable [PR112581]

Like r14-2293-g11350734240dba and r14-2289-gb083203f053f16,
reassociation can combine across a few bb and one of the usage
can be an uninitializated variable and if going from an conditional
usage to an unconditional usage can cause wrong code.
This uses maybe_undef_p like other passes where this can happen.

Note if-to-switch uses the function (init_range_entry) provided
by ressociation so we need to call mark_ssa_maybe_undefs there;
otherwise we assume almost all ssa names are uninitialized.

Bootstrapped and tested on x86_64-linux-gnu.

gcc/ChangeLog:

	PR tree-optimization/112581
	* gimple-if-to-switch.cc (pass_if_to_switch::execute): Call
	mark_ssa_maybe_undefs.
	* tree-ssa-reassoc.cc (can_reassociate_op_p): Uninitialized
	variables can not be reassociated.
	(init_range_entry): Check for uninitialized variables too.
	(init_reassoc): Call mark_ssa_maybe_undefs.

gcc/testsuite/ChangeLog:

	PR tree-optimization/112581
	* gcc.c-torture/execute/pr112581-1.c: New test.

Signed-off-by: Andrew Pinski <quic_apinski@quicinc.com>