[LV] Mark dead instructions in loop as free.

Update collectValuesToIgnore to also ignore dead instructions in the
loop. Such instructions will be removed by VPlan-based DCE and won't be
considered by the VPlan-based cost model.

This closes a gap between the legacy and VPlan-based cost model. In
practice with the default pipelines, there shouldn't be any dead
instructions in loops reaching LoopVectorize, but it is easy to generate
such cases by hand or automatically via fuzzers.

Fixes https://github.com/llvm/llvm-project/issues/99701.

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