match.pd: Fix up __builtin_mul_overflow_p signed type optimization [PR105984]

Earlier in the simplification pattern, we require that @0 has compatible
type to the type of IMAGPART_EXPR, but for @1 which is a non-zero constant
all we require is that it the constant fits into that type.
Later the code checks if the constant is negative, because when min / max
values are divided by negative divisor, lo will be higher than hi.
In the following testcase, @1 has unsigned char type, while @0 has
int type, so @1 which is 254 is wi::neg_p and we were swapping lo and hi,
even when @1 cast to int isn't negative.

We could use tree_int_cst_sgn (@1) < 0 as the check instead and it would
work both for narrower types of @1 and even same or wider ones, but
I've noticed we probably don't want to call fold_convert (TREE_TYPE (@0), @1)
twice and when we save that result in a temporary, we can just use wi::neg_p
on that temporary.

2022-06-16  Jakub Jelinek  <jakub@redhat.com>

	PR tree-optimization/105984
	* match.pd (__builtin_mul_overflow_p (x, cst, (stype) 0) ->
	x > stype_max / cst || x < stype_min / cst): fold_convert @1
	to TREE_TYPE (@0) just once and test for negative divisor
	also on that folded constant instead of on @1.

	* gcc.c-torture/execute/pr105984.c: New test.

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