[NFC][InstructionCost] Remove isValid() asserts in SLPVectorizer.cpp
An earlier patch introduced asserts that the InstructionCost is valid because at that time the ReuseShuffleCost variable was an unsigned. However, now that the variable is an InstructionCost instance the asserts can be removed.
See this thread for context: http://lists.llvm.org/pipermail/llvm-dev/2020-November/146408.html
See this patch for the introduction of the type: https://reviews.llvm.org/D91174