try simplifying disabled_loop_vectorization test fn to constant store

Author: GYT

llvm/test/Transforms/LoopVectorize/diag-disabled-vectorization-msgs.ll

@@ -47,18 +47,14 @@ entry:
 
 loop:
   %iv = phi i64 [ 0, %entry ], [ %inc, %loop ]
-  %rdx = phi double [ 0.000000e+00, %entry ], [ %add, %loop ]
   %arrayidx = getelementptr inbounds nuw double, ptr %src, i64 %iv
-  %1 = load double, ptr %arrayidx, align 8
-  %sub = fsub fast double %1, 1.234e+0
-  %mul = fmul fast double %sub, %sub
-  %add = fadd fast double %mul, %rdx
+  store double 1.234e+0, ptr %arrayidx, align 8
   %inc = add nuw nsw i64 %iv, 1
   %exitcond.not = icmp eq i64 %inc, 15
   br i1 %exitcond.not, label %exit, label %loop, !llvm.loop !0
 
 exit:
-  ret double %add
+  ret i64 0
 }
 
 !0 = distinct !{!0, !1}