[SLPVectorizer] Avoid duplicate scalar cost calculations in BoUpSLP::getEntryCost. NFCI.
[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: 0ff597fd157e6f4fc38580e8d64ab130330d2411@337390 91177308-0d34-0410-b5e6-96231b3b80d8

GPUOpen-Drivers/llvm,llvm-mirror/llvm,GPUOpen-Drivers/llvm,llvm-mirror/llvm,llvm-mirror/llvm,GPUOpen-Drivers/llvm,llvm-mirror/llvm,apple/swift-llvm,GPUOpen-Drivers/llvm,apple/swift-llvm,apple/swift-llvm,GPUOpen-Drivers/llvm,GPUOpen-Drivers/llvm,apple/swift-llvm,apple/swift-llvm,llvm-mirror/llvm,llvm-mirror/llvm,apple/swift-llvm,GPUOpen-Drivers/llvm,llvm-mirror/llvm,llvm-mirror/llvm,llvm-mirror/llvm,apple/swift-llvm,GPUOpen-Drivers/llvm,apple/swift-llvm