fix vectorization analysis msg for runtime checks

[milthorpe]

Summary

Update analysis message to indicate that the loop was not vectorized
due to unprofitability from too many required runtime checks, not
because it is unsafe to do so.

[milthorpe]

Update vectorization analysis msg in clang frontend test

clang/test/Frontend/optimization-remark-options.c

 // REQUIRES: x86-registered-target
 // RUN: %clang -O1 -fvectorize -target x86_64-unknown-unknown -mllvm -vectorize-memory-check-threshold=8 -Rpass-analysis=loop-vectorize -emit-llvm -S %s -o - 2>&1 | FileCheck %s
 
 // CHECK: {{.*}}:10:11: remark: loop not vectorized: cannot prove it is safe to reorder floating-point operations; allow reordering by specifying '#pragma clang loop vectorize(enable)' before the loop or by providing the compiler option '-ffast-math'.
 
 double foo(int N) {
   double v = 0.0;
 
   for (int i = 0; i < N; i++)
     v = v + 1.0;
 
   return v;
 }
 
-// CHECK: {{.*}}:18:3: remark: loop not vectorized: cannot prove it is safe to reorder memory operations; allow reordering by specifying '#pragma clang loop vectorize(enable)' before the loop. If the arrays will always be independent specify '#pragma clang loop vectorize(assume_safety)' before the loop or provide the '__restrict__' qualifier with the independent array arguments. Erroneous results will occur if these options are incorrectly applied!
+// CHECK: {{.*}}:18:3: remark: loop not vectorized: it is not profitable to reorder memory operations with runtime checks; allow reordering by specifying '#pragma clang loop vectorize(enable)' before the loop. If the arrays will always be independent specify '#pragma clang loop vectorize(assume_safety)' before the loop or provide the '__restrict__' qualifier with the independent array arguments. Erroneous results will occur if these options are incorrectly applied!
 
 void foo2(int *dw, int *uw, int *A, int *B, int *C, int *D, int N) {
   for (long i = 0; i < N; i++) {
     dw[i] = A[i] + B[i - 1] + C[i - 2] + D[i - 3];
     uw[i] = A[i] + B[i + 1] + C[i + 2] + D[i + 3];
   }
 }

llvm/lib/Transforms/Vectorize/LoopVectorize.cpp

bool ForceVectorization =
         Hints.getForce() == LoopVectorizeHints::FK_Enabled;
     if (!ForceVectorization &&
         !areRuntimeChecksProfitable(Checks, VF, CM.getVScaleForTuning(), L,
                                     *PSE.getSE())) {
       ORE->emit([&]() {
         return OptimizationRemarkAnalysisAliasing(
                    DEBUG_TYPE, "CantReorderMemOps", L->getStartLoc(),
                    L->getHeader())
-               << "loop not vectorized: cannot prove it is safe to reorder "
-                  "memory operations";
+               << "loop not vectorized: it is not profitable to reorder memory "
+                  "operations with runtime checks";
       });
       LLVM_DEBUG(dbgs() << "LV: Too many memory checks needed.\n");
       Hints.emitRemarkWithHints();
       return false;
     }
   }
 
   // Identify the diagnostic messages that should be produced.