Fix PR 110386: backprop vs ABSU_EXPR

Fix PR 110386: backprop vs ABSU_EXPR

The issue here is that when backprop tries to go
and strip sign ops, it skips over ABSU_EXPR but
ABSU_EXPR not only does an ABS, it also changes the
type to unsigned.
Since strip_sign_op_1 is only supposed to strip off
sign changing operands and not ones that change types,
removing ABSU_EXPR here is correct. We don't handle
nop conversions so this does cause any missed optimizations either.

OK? Bootstrapped and tested on x86_64-linux-gnu with no regressions.

	PR tree-optimization/110386

gcc/ChangeLog:

	* gimple-ssa-backprop.cc (strip_sign_op_1): Remove ABSU_EXPR.

gcc/testsuite/ChangeLog:

	* gcc.c-torture/compile/pr110386-1.c: New test.
	* gcc.c-torture/compile/pr110386-2.c: New test.

(cherry picked from commit 2bbac12ea7bd8a3eef5382e1b13f6019df4ec03f)