Make sure -optimize-regalloc=false is used correctly by user.

Don't allow -optimize-regalloc=false with -regalloc given for anything other
than 'fast'. The other register allocators depend on the supporting passes
added by addOptimizedRegAlloc().

Reviewers: Quentin Colombet, Matthias Braun
https://reviews.llvm.org/D33181

llvm-svn: 303238