Remove dead code. Improve llvm_unreachable text. Simplify some control flow.

git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@150918 91177308-0d34-0410-b5e6-96231b3b80d8

include/llvm/Analysis/ScalarEvolutionExpressions.h

@@ -491,7 +491,6 @@ namespace llvm {
 
     RetVal visitCouldNotCompute(const SCEVCouldNotCompute *S) {
       llvm_unreachable("Invalid use of SCEVCouldNotCompute!");
-      return RetVal();
     }
   };
 }