Revert r195317 (and r195333), "Teach ISel not to optimize 'optnone' functions."

It broke, at least, i686 target. It is reproducible with "llc -mtriple=i686-unknown".

FYI, it didn't appear to add either "-O0" or "-fast-isel".

git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@195339 91177308-0d34-0410-b5e6-96231b3b80d8