More NEGATE_EXPR folding in match.pd

As observed by Jakub in comment #2 of PR 98865, the expression -(a>>63)
is optimized in GENERIC but not in GIMPLE.  Investigating further it
turns out that this is one of a few transformations performed by
fold_negate_expr in fold-const.c that aren't yet performed by match.pd.
This patch moves/duplicates them there, and should be relatively safe
as these transformations are already performed by the compiler, but
just in different passes.

This revised patch adds a Boolean simplify argument to tree-ssa-sccvn.c's
vn_nary_build_or_lookup_1 to control whether simplification should be
performed before value numbering, updating the callers, but then
avoiding simplification when constructing/value-numbering NEGATE_EXPR.
This avoids the regression of gcc.dg/tree-ssa/ssa-free-88.c, and enables
the new test case(s) to pass.

2021-09-22  Roger Sayle  <roger@nextmovesoftware.com>
	    Richard Biener  <rguenther@suse.de>

gcc/ChangeLog
	* match.pd (negation simplifications): Implement some negation
	folding transformations from fold-const.c's fold_negate_expr.
	* tree-ssa-sccvn.c (vn_nary_build_or_lookup_1): Add a SIMPLIFY
	argument, to control whether the op should be simplified prior
	to looking up/assigning a value number.
	(vn_nary_build_or_lookup): Update call to vn_nary_build_or_lookup_1.
	(vn_nary_simplify): Likewise.
	(visit_nary_op): Likewise, but when constructing a NEGATE_EXPR
	now call vn_nary_build_or_lookup_1 disabling simplification.

gcc/testsuite/ChangeLog
	* gcc.dg/fold-negate-1.c: New test case.

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