[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