New version that passed bootstrap+regtest during the night.

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.

I am a bit confused that with avx512 we get types like "vector(4) 
<signed-boolean:2>" with :2 and not :1 (is it a hack so true is 1 and not 
-1?), but that doesn't matter for this patch.

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.

-- 
Marc Glisse