[LV] Don't let ForceTargetInstructionCost override Invalid cost.

Invalid costs can be used to avoid vectorization with a given VF, which is
used for scalable vectors to avoid things that the code-generator cannot
handle. If we override the cost using the -force-target-instruction-cost
option of the LV, we would override this mechanism, rendering the flag useless.

This change ensures the cost is only overriden when the original cost that
was calculated is valid. That allows the flag to be used in combination
with the -scalable-vectorization option.

Reviewed By: david-arm

Differential Revision: https://reviews.llvm.org/D106677