[SCEV][NFC] Remove duplication of isLoopInvariant code

[mkazantsev]

Summary

Replace two places that duplicate the code of isLoopInvariant method with
the invocation of this method.

lib/Analysis/ScalarEvolution.cpp

static const SCEV *rewrite(const SCEV *S, const Loop *L,
     const SCEV *Result = Rewriter.visit(S);
     return Rewriter.isValid() ? Result : SE.getCouldNotCompute();
   }
 
   SCEVInitRewriter(const Loop *L, ScalarEvolution &SE)
       : SCEVRewriteVisitor(SE), L(L), Valid(true) {}
 
   const SCEV *visitUnknown(const SCEVUnknown *Expr) {
-    if (!(SE.getLoopDisposition(Expr, L) == ScalarEvolution::LoopInvariant))
+    if (!SE.isLoopInvariant(Expr, L))
       Valid = false;
     return Expr;
   }
 
   const SCEV *visitAddRecExpr(const SCEVAddRecExpr *Expr) {
     // Only allow AddRecExprs for this loop.
     if (Expr->getLoop() == L)
       return Expr->getStart();
static const SCEV *rewrite(const SCEV *S, const Loop *L,
     return Rewriter.isValid() ? Result : SE.getCouldNotCompute();
   }
 
   SCEVShiftRewriter(const Loop *L, ScalarEvolution &SE)
       : SCEVRewriteVisitor(SE), L(L), Valid(true) {}
 
   const SCEV *visitUnknown(const SCEVUnknown *Expr) {
     // Only allow AddRecExprs for this loop.
-    if (!(SE.getLoopDisposition(Expr, L) == ScalarEvolution::LoopInvariant))
+    if (!SE.isLoopInvariant(Expr, L))
       Valid = false;
     return Expr;
   }
 
   const SCEV *visitAddRecExpr(const SCEVAddRecExpr *Expr) {
     if (Expr->getLoop() == L && Expr->isAffine())
       return SE.getMinusSCEV(Expr, Expr->getStepRecurrence(SE));
     Valid = false;