[SLPVectorizer] Replace VL[0] to VL0 with assert, add propagateIRFlags extra parameter VL0,                  replace E->Scalars[0] to VL0, NFCI.
[SLPVectorizer] Replace VL[0] to VL0 with assert, add propagateIRFlags extra parameter VL0, 
                replace E->Scalars[0] to VL0, NFCI.


git-svn-id: 0ff597fd157e6f4fc38580e8d64ab130330d2411@310904 91177308-0d34-0410-b5e6-96231b3b80d8

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