[PATCH][LoopStrengthReduce] Don't trim a formula that uses a subset of required registers

[Robinson, Paul]

> > > * remove clang from the substitutions in lit.cfg so that the next 
> > >   person won't get confused like I got
> >
> > Sure, but in a separate commit.
> >
> Of course.

While you're there, please also remove c-index-test from the list.
I'm pretty sure the rest of the tools are actually built by LLVM,
although the acid test would be a just-llvm checkout and build.

I did a find+grep of llvm/test and found no uses of 'RUN:.*clang' or
'RUN:.*c-index-test' so it should be okay.
--paulr