[VPlan] Move recording of Inst->VPValue to VPRecipeBuilder (NFCI). (#84464)

Instead of keeping a mapping of Inst->VPValues (of their corresponding
recipes) in VPlan's Value2VPValue mapping, keep it in VPRecipeBuilder
instead. After recently replacing the last user of this mapping after
initial construction, this mapping is only needed for recipe
construction (to map IR operands to VPValue operands).

By moving the mapping, VPlan's VPValue tracking can be simplified and
limited only to live-ins. It also allows removing disableValue2VPValue
and associated machinery & asserts.

PR: https://github.com/llvm/llvm-project/pull/84464