[irange] Return FALSE if updated bitmask is unchanged [PR110753]

[irange] Return FALSE if updated bitmask is unchanged [PR110753]

The mask/value pair we track in the irange is a bit fickle in that it
can sometimes contradict the bitmask inherent in the range.  This can
happen when a series of calculations yield a combination such as:

	[3, 1000] MASK 0xfffffffe VALUE 0x0

The mask/value above implies that the lowest bit is a known 0, which
would exclude the 3 in the range.  At one time we tried keeping mask
and ranges 100% consistent, but the performance penalty was too high
(5% in VRP).  Also, it's unclear whether the intersection of two
incompatible known bits should make the whole range undefined, or
just the contradicting bits.  This is all documented in
irange::get_bitmask().  We could revisit both of these assumptions
in the future.

In this testcase IPA ends up with a range where the lower 2 bits are
expected to be 0, but the range is [1,1].

	[irange] long int [1, 1] MASK 0xfffffffffffffffc VALUE 0x0

This causes irange::union_bitmask() to think an update occurred, when
no semantic change happened, thus triggering an assert in IPA-cp.  We
could get rid of the assert, but it's cleaner to make
irange::{union,intersect}_bitmask always tell the truth.  Beside, the
ranger's cache also depends on union being truthful.

	PR ipa/110753

gcc/ChangeLog:

	* value-range.cc (irange::union_bitmask): Return FALSE if updated
	bitmask is semantically equivalent to the original mask.
	(irange::intersect_bitmask): Same.
	(irange::get_bitmask): Add comment.

gcc/testsuite/ChangeLog:

	* gcc.dg/tree-ssa/pr110753.c: New test.