[CodeGenPrepare] Exit earlier when optimizing selects (NFC)
[CodeGenPrepare] Exit earlier when optimizing selects (NFC)

When optimizing for size, this allows optimizeSelectInst to skip a
linear scan and exit early.

git-svn-id: 0ff597fd157e6f4fc38580e8d64ab130330d2411@340367 91177308-0d34-0410-b5e6-96231b3b80d8
(cherry picked from commit 510d469d61b27b0ffd9354e140314aa4667f63d2)

apple/swift-llvm,apple/swift-llvm,apple/swift-llvm,apple/swift-llvm,apple/swift-llvm,apple/swift-llvm,apple/swift-llvm,apple/swift-llvm