[VPlan] Only generate single instr for unpredicated stores of varying value to invariant address

This extends the previously added uniform store case to handle stores of loop varying values to a loop invariant address. Note that the placement of this code only allows unpredicated stores; this is important for correctness. (That is "IsPredicated" is always false at this point in the function.)

This patch does not include scalable types. The diff felt "large enough" as it were; I'll handle that in a separate patch. (It requires some changes to cost modeling.)

Differential Revision: https://reviews.llvm.org/D133580