[PR86153] simplify more overflow tests in VRP

[PR86153] simplify more overflow tests in VRP

PR 86153 was originally filed when changes to the C++11's
implementation of vector resize(size_type) limited inlining that were
required for testsuite/g++.dg/pr83239.C to verify that we did not
issue an undesired warning.

That was worked by increasing the limit for inlining, but that in turn
caused the C++98 implementation of vector resize, that is
significantly different, to also be fully inlined, and that happened
to issue the very warnings the test was meant to verify we did NOT
issue.

The reason we issued the warnings was that we failed to optimize out
some parts of _M_fill_insert, used by the C++98 version of vector
resize, although the call of _M_fill_insert was guarded by a test that
could never pass: test testcase only calls resize when the vector size
is >= 3, to decrement the size by two.  The limitation we hit in VRP
was that the compared values could pass as an overflow test, if the
vector size was 0 or 1 (we knew it wasn't), but even with dynamic
ranges we failed to decide that the test result could be determined at
compile time, even though after the test we introduced ASSERT_EXPRs
that required a condition known to be false from earlier ones.

I pondered turning ASSERT_EXPRs that show impossible conditions into
traps, to enable subsequent instructions to be optimized, but I ended
up finding an earlier spot in which an overflow test that would have
introduced the impossible ASSERT_EXPR can have its result deduced from
earlier known ranges and resolved to the other path.

Although such overflow tests could be uniformly simplified to compares
against a constant, the original code would only perform such
simplifications when the test could be resolved to an equality test
against zero.  I've thus avoided introducing compares against other
constants, and instead added code that will only simplify overflow
tests that weren't simplified before when the condition can be
evaluated at compile time.

for  gcc/ChangeLog

	PR testsuite/86153
	PR middle-end/83239
	* vr-values.c
	(vr_values::vrp_evaluate_conditional_warnv_with_ops): Extend
	simplification of overflow tests to cover cases in which we
	can determine the result of the comparison.

for  gcc/testsuite/ChangeLog

	PR testsuite/86153
	PR middle-end/83239
	* gcc.dg/vrp-overflow-1.c: New.

From-SVN: r267252