[Analysis] simplify code for getSplatValue(); NFC
[Analysis] simplify code for getSplatValue(); NFC

AFAIK, this is only currently called by TTI, but it could be
used from instcombine or CGP to help solve problems like:
https://bugs.llvm.org/show_bug.cgi?id=37428
https://bugs.llvm.org/show_bug.cgi?id=42174

git-svn-id: 0ff597fd157e6f4fc38580e8d64ab130330d2411@362810 91177308-0d34-0410-b5e6-96231b3b80d8

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