Update the legacy cost model skip branches with successors blocks
that are empty or only contain dead instructions, together with their
conditions. Such branches and conditions won't result in any
generated code and will be cleaned up by VPlan transforms.
This fixes a difference between the legacy and VPlan-based cost model.
When running LV in its usual pipeline position, such dead blocks should
already have been cleaned up, but they might be generated manually or by
fuzzers.
Fixes https://github.com/llvm/llvm-project/issues/100591.
e9e3a183d6