[VectorCombine] Don't transform single shuffles in shuffleToIdentity

This will help in later patches where the checks for operands being
instructions is removed, and might help not remove unnecessary poison lanes.

Author: davemgreen

llvm/lib/Transforms/Vectorize/VectorCombine.cpp

@@ -1794,6 +1794,9 @@ bool VectorCombine::foldShuffleToIdentity(Instruction &I) {
     }
   }
 
+  if (NumVisited <= 1)
+    return false;
+
   // If we got this far, we know the shuffles are superfluous and can be
   // removed. Scan through again and generate the new tree of instructions.
   std::function<Value *(ArrayRef<InstLane>)> Generate =

llvm/test/Transforms/VectorCombine/X86/select-shuffle.ll

@@ -12,11 +12,12 @@ define <4 x double> @PR60649() {
 ; CHECK:       unreachable:
 ; CHECK-NEXT:    br label [[END]]
 ; CHECK:       end:
-; CHECK-NEXT:    [[TMP0:%.*]] = phi <4 x double> [ zeroinitializer, [[ENTRY:%.*]] ], [ zeroinitializer, [[UNREACHABLE:%.*]] ]
+; CHECK-NEXT:    [[T0:%.*]] = phi <4 x double> [ zeroinitializer, [[ENTRY:%.*]] ], [ zeroinitializer, [[UNREACHABLE:%.*]] ]
 ; CHECK-NEXT:    [[T1:%.*]] = phi <4 x double> [ zeroinitializer, [[ENTRY]] ], [ zeroinitializer, [[UNREACHABLE]] ]
-; CHECK-NEXT:    [[TMP1:%.*]] = shufflevector <4 x double> [[TMP0]], <4 x double> [[TMP0]], <4 x i32> <i32 2, i32 3, i32 poison, i32 poison>
-; CHECK-NEXT:    [[TMP2:%.*]] = fdiv <4 x double> [[TMP0]], <double 0.000000e+00, double 0.000000e+00, double undef, double undef>
-; CHECK-NEXT:    [[TMP3:%.*]] = fmul <4 x double> [[TMP1]], <double 0.000000e+00, double 0.000000e+00, double undef, double undef>
+; CHECK-NEXT:    [[TMP0:%.*]] = shufflevector <4 x double> [[T0]], <4 x double> [[T0]], <4 x i32> <i32 2, i32 3, i32 poison, i32 poison>
+; CHECK-NEXT:    [[TMP1:%.*]] = shufflevector <4 x double> [[T0]], <4 x double> [[T0]], <4 x i32> <i32 0, i32 1, i32 poison, i32 poison>
+; CHECK-NEXT:    [[TMP2:%.*]] = fdiv <4 x double> [[TMP1]], <double 0.000000e+00, double 0.000000e+00, double undef, double undef>
+; CHECK-NEXT:    [[TMP3:%.*]] = fmul <4 x double> [[TMP0]], <double 0.000000e+00, double 0.000000e+00, double undef, double undef>
 ; CHECK-NEXT:    [[T5:%.*]] = shufflevector <4 x double> [[TMP2]], <4 x double> [[TMP3]], <4 x i32> <i32 0, i32 1, i32 4, i32 5>
 ; CHECK-NEXT:    ret <4 x double> [[T5]]
 ;