Improve handling of unknown sign bit in CCP.

Improve handling of unknown sign bit in CCP.

This middle-end patch implements several related improvements to
tree-ssa's conditional (bit) constant propagation pass.  The current
code handling ordered comparisons contains the comment "If the
most significant bits are not known we know nothing" which is not
entirely true [this test even prevents this pass understanding these
comparisons always have a zero or one result].  This patch introduces
a new value_mask_to_min_max helper function, that understands the
different semantics of the most significant bit on signed vs.
unsigned values.  This allows us to generalize ordered comparisons,
GE_EXPR, GT_EXPR, LE_EXPR and LT_EXPR, where to code is tweaked to
correctly handle the potential equal cases.  Then finally support
is added for the related tree codes MIN_EXPR, MAX_EXPR, ABS_EXPR
and ABSU_EXPR.

Regression testing revealed three test cases in the testsuite that
were checking for specific optimizations that are now being performed
earlier than expected.  These tests can continue to check their
original transformations by explicitly adding -fno-tree-ccp to their
dg-options (some already specify -fno-ipa-vrp or -fno-tree-forwprop
for the same reason).

2021-08-09  Roger Sayle  <roger@nextmovesoftware.com>

gcc/ChangeLog
	* tree-ssa-ccp.c (value_mask_to_min_max): Helper function to
	determine the upper and lower bounds from a mask-value pair.
	(bit_value_unop) [ABS_EXPR, ABSU_EXPR]: Add support for
	absolute value and unsigned absolute value expressions.
	(bit_value_binop):  Initialize *VAL's precision.
	[LT_EXPR, LE_EXPR]: Use value_mask_to_min_max to determine
	upper and lower bounds of operands.  Add LE_EXPR/GE_EXPR
	support when the operands are unknown but potentially equal.
	[MIN_EXPR, MAX_EXPR]: Support minimum/maximum expressions.

gcc/testsuite/ChangeLog
	* gcc.dg/pr68217.c: Add -fno-tree-ccp option.
	* gcc.dg/tree-ssa/vrp24.c: Add -fno-tree-ccp option.
	* g++.dg/ipa/pure-const-3.C: Add -fno-tree-ccp option.