[LV] Skip cost for ZExt/SExts that will be removed by truncating ops.
If an extend is truncated, it will be removed if the result type is <=
the source type, as there is nothing to extend. Return a cost of 0.
This was caught by the first step to perform cost-modeling based on
VPlan (b841e2e), as the legacy cost model would query the cost of an
invalid extend, while the extend has been folded away by VPlan
transforms.
Fixes https://github.com/llvm/llvm-project/issues/98413.
llvm-project
67f4968a - [LV] Skip cost for ZExt/SExts that will be removed by truncating ops.
