[CodeGenPrepare] Scan past debug intrinsics to find select candidates (NFC)
[CodeGenPrepare] Scan past debug intrinsics to find select candidates (NFC)

In optimizeSelectInst, when scanning for candidate selects to rewrite
into branches, scan past debug intrinsics. This makes the debug-enabled
and non-debug paths through optimizeSelectInst more congruent.

NFC because every select is eventually visited either way.

git-svn-id: 0ff597fd157e6f4fc38580e8d64ab130330d2411@340368 91177308-0d34-0410-b5e6-96231b3b80d8
(cherry picked from commit 404a8add81478c213bec9399604041de41e916b3)

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