Teach ISel not to optimize 'optnone' functions (revised).

Teach ISel not to optimize 'optnone' functions (revised).

Improvements over r195317:
- Set/restore EnableFastISel flag instead of just running FastISel within
  SelectAllBasicBlocks; the flag is checked in various places, and
  FastISel won't run properly if those places don't do the right thing.
- Test looks for normal ISel versus FastISel behavior, and not
  something more subtle that doesn't work everywhere.

Based on work by Andrea Di Biagio.

git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@195491 91177308-0d34-0410-b5e6-96231b3b80d8