[VPlan] Make CallInst optional for VPWidenCallRecipe (NFCI).

Replace relying on the underling CallInst for looking up the called
function and its types by instead adding the called function as operand,
in line with how called functions are handled in CallInst.

Operand bundles, metadata and fast-math flags are optionally used if
there's an underlying CallInst.

This enables creating VPWidenCallRecipes without requiring an underlying
IR instruction.