If a phi is widened with tail folding, all of its predecessors will have
a mask of the form
%x = logical-and %active-lane-mask, %foo
%y = logical-and %active-lane-mask, %bar
%z = logical-and %active-lane-mask, %baz
...
We can remove the common %active-lane-mask from all of these edge masks,
which in turn allows us to simplify a lot of VPBlendRecipes.
In particular, it allows the header mask to be removed in selects with
EVL tail folding, improving RISC-V codegen on SPEC CPU 2017 for
525.x264_r, and supersedes #147243.
This also allows us to remove VPBlendRecipe and directly emit
VPInstruction::Select in another patch.
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