VEC_COND_EXPR optimizations

When vector comparisons were forced to use vec_cond_expr, we lost a number of optimizations (my fault for not adding enough testcases to
prevent that). This patch tries to unwrap vec_cond_expr a bit so some optimizations can still happen.

I wasn't planning to add all those transformations together, but adding one caused a regression, whose fix introduced a second regression,
etc.

Restricting to constant folding would not be sufficient, we also need at least things like X|0 or X&X. The transformations are quite
conservative with :s and folding only if everything simplifies, we may want to relax this later. And of course we are going to miss things
like a?b:c + a?c:b -> b+c.

In terms of number of operations, some transformations turning 2 VEC_COND_EXPR into VEC_COND_EXPR + BIT_IOR_EXPR + BIT_NOT_EXPR might not look
like a gain... I expect the bit_not disappears in most cases, and VEC_COND_EXPR looks more costly than a simpler BIT_IOR_EXPR.

2020-08-05  Marc Glisse  <marc.glisse@inria.fr>

	PR tree-optimization/95906
	PR target/70314
	* match.pd ((c ? a : b) op d, (c ? a : b) op (c ? d : e),
	(v ? w : 0) ? a : b, c1 ? c2 ? a : b : b): New transformations.
	(op (c ? a : b)): Update to match the new transformations.

	* gcc.dg/tree-ssa/andnot-2.c: New file.
	* gcc.dg/tree-ssa/pr95906.c: Likewise.
	* gcc.target/i386/pr70314.c: Likewise.

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