Change SelectCode's argument from SDValue to SDNode *, to make it more

Change SelectCode's argument from SDValue to SDNode *, to make it more
clear what information these functions are actually using.

This is also a micro-optimization, as passing a SDNode * around is
simpler than passing a { SDNode *, int } by value or reference.

git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@92564 91177308-0d34-0410-b5e6-96231b3b80d8