[CodeGen] Some assorted cleanups
[CodeGen] Some assorted cleanups

No functional change, just some cleanups:
- Use auto when it is appropriate.
- There were some strange static_casts which were superfluous.
- Use range-based for loops when appropriate.
- The dyn_cast_or_null construct was used when null was impossible.

git-svn-id: ffe668792ed300d6c2daa1f6eba2e0aa28d7ec6c@275699 91177308-0d34-0410-b5e6-96231b3b80d8

llvm-mirror/clang,apple/swift-clang,llvm-mirror/clang,llvm-mirror/clang,llvm-mirror/clang,llvm-mirror/clang,llvm-mirror/clang,apple/swift-clang,apple/swift-clang,llvm-mirror/clang,apple/swift-clang,apple/swift-clang,llvm-mirror/clang,apple/swift-clang,apple/swift-clang,llvm-mirror/clang,apple/swift-clang,apple/swift-clang,apple/swift-clang,llvm-mirror/clang