[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: https://llvm.org/svn/llvm-project/llvm/trunk@340367 91177308-0d34-0410-b5e6-96231b3b80d8
1 file changed