match.pd: Optimize ~(X >> Y) to ~X >> Y if ~X can be simplified [PR96688]

match.pd: Optimize ~(X >> Y) to ~X >> Y if ~X can be simplified [PR96688]

This patch optimizes two GIMPLE operations into just one.
As mentioned in the PR, there is some risk this might create more expensive
constants, but sometimes it will make them on the other side less expensive,
it really depends on the exact value.
And if it is an important issue, we should do it in md or during expansion.

2021-01-14  Jakub Jelinek  <jakub@redhat.com>

	PR tree-optimization/96688
	* match.pd (~(X >> Y) -> ~X >> Y): New simplification if
	~X can be simplified.

	* gcc.dg/tree-ssa/pr96688.c: New test.
	* gcc.dg/tree-ssa/reassoc-37.c: Adjust scan-tree-dump regex.
	* gcc.target/i386/pr66821.c: Likewise.