Since the new instruction selector now works, I don't need to keep

Since the new instruction selector now works, I don't need to keep
the old one around for comparative purposes: have the
ENABLE_NEW_ISEL #define (which is not enabled on mainline) stop
emitting the old isel at all, yay for build time win.

git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@97033 91177308-0d34-0410-b5e6-96231b3b80d8