[SLPVectorizer] Avoid duplicate scalar cost calculations in BoUpSLP::getEntryCost. NFCI.

Pulled out from D49225, we have a lot of repeated scalar cost calculations, often with arguments that don't look the same but turn out to be.

git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@337390 91177308-0d34-0410-b5e6-96231b3b80d8
1 file changed