VPRecipeBuilder::createBlockInMask currently directly uses
Legal->getPrimaryInduction(). This tightly couples mask code-generation
with Legal. This patch introduces a new PrimaryIV VPValue to the VPlan
(similar to the BackedgeTakenCount), which can be used for masking,
instead of querying legal directly.
This uncouples code-generation from Legal and allows for changing the
primary IV before code-generation.