This patch adds VPValue version of the instruction operands to
VPWidenRecipe and uses them during code-generation. It moves the code
generation from InnerLoopVectorizer::widenInstruction to
VPWidenRecipe::execute, so we can conveniently access the underlying
values of VPValues.
Similar to D76373 this reduces ingredient def-use usage by ILV as
a step towards full VPlan-based def-use relations.