This patch replaces VPWidenRecipe with VPInstructions. This is still WIP
and needs to handle a few additional cases, but I wanted to share this
early to make sure the direction makes sense.
With the current version, we get
Expected Passes : 286 Unexpected Failures: 63
I introduced a new VPWidenInstruction, but maybe it would be better to
just use VPInstruction.
If the direction looks right, I'll also continue with other recipes.