[IndVarSimplify] Simplify code using preheader assumption

As noted in the nice block comment, the previous code didn't actually handle multi-entry loops correctly, it just assumed SCEV didn't analyze such loops.  Given SCEV has comments to the contrary, that seems a bit suspect.  More importantly, the pass actually requires loopsimplify form which ensures a loop-preheader is available.  Remove the excessive generaility and shorten the code greatly.

Note that we do successfully analyze many multi-entry loops, but we do so by converting them to single entry loops.  See the added test case.

llvm-svn: 316976