simplify-rtx: Optimize (x - 1) * y + y [PR98334]

simplify-rtx: Optimize (x - 1) * y + y [PR98334]

We don't try to optimize for signed x, y (int) (x - 1U) * y + y
into x * y, we can't do that with signed x * y, because the former
is well defined for INT_MIN and -1, while the latter is not.
We could perhaps optimize it during isel or some very late optimization
where we'd turn magically flag_wrapv, but we don't do that yet.

This patch optimizes it in simplify-rtx.c, such that we can optimize
it during combine.

2021-01-05  Jakub Jelinek  <jakub@redhat.com>

	PR rtl-optimization/98334
	* simplify-rtx.c (simplify_context::simplify_binary_operation_1):
	Optimize (X - 1) * Y + Y to X * Y or (X + 1) * Y - Y to X * Y.

	* gcc.target/i386/pr98334.c: New test.