[VectorCombine] Add constant splat handling for shuffleToIdentity

This adds splat constants, which can be treated like any other splat which
makes them very simple. Other constants are not handled yet, they are planned
in a later patch.

llvm/lib/Transforms/Vectorize/VectorCombine.cpp

@@ -1789,6 +1789,17 @@ bool VectorCombine::foldShuffleToIdentity(Instruction &I) {
       IdentityLeafs.insert(FrontV);
       continue;
     }
+    // Look for constants, for the moment only supporting constant splats.
+    if (auto *C = dyn_cast<Constant>(FrontV);
+        C && C->getSplatValue() &&
+        all_of(drop_begin(Item), [Item](InstLane &IL) {
+          Value *FrontV = Item.front().first;
+          Value *V = IL.first;
+          return !V || V == FrontV;
+        })) {
+      SplatLeafs.insert(FrontV);
+      continue;
+    }
     // Look for a splat value.
     if (all_of(drop_begin(Item), [Item](InstLane &IL) {
           auto [FrontV, FrontLane] = Item.front();

llvm/test/Transforms/VectorCombine/AArch64/shuffletoidentity.ll

@@ -281,11 +281,7 @@ define <8 x i8> @undeflane(<8 x i8> %a, <8 x i8> %b) {
 
 define <8 x i8> @constantsplat(<8 x i8> %a) {
 ; CHECK-LABEL: @constantsplat(
-; CHECK-NEXT:    [[AB:%.*]] = shufflevector <8 x i8> [[A:%.*]], <8 x i8> poison, <4 x i32> <i32 3, i32 2, i32 1, i32 0>
-; CHECK-NEXT:    [[AT:%.*]] = shufflevector <8 x i8> [[A]], <8 x i8> poison, <4 x i32> <i32 7, i32 6, i32 5, i32 4>
-; CHECK-NEXT:    [[ABT:%.*]] = add <4 x i8> [[AT]], <i8 10, i8 10, i8 10, i8 10>
-; CHECK-NEXT:    [[ABB:%.*]] = add <4 x i8> [[AB]], <i8 10, i8 10, i8 10, i8 10>
-; CHECK-NEXT:    [[R:%.*]] = shufflevector <4 x i8> [[ABT]], <4 x i8> [[ABB]], <8 x i32> <i32 7, i32 6, i32 5, i32 4, i32 3, i32 2, i32 1, i32 0>
+; CHECK-NEXT:    [[R:%.*]] = add <8 x i8> [[A:%.*]], <i8 10, i8 10, i8 10, i8 10, i8 10, i8 10, i8 10, i8 10>
 ; CHECK-NEXT:    ret <8 x i8> [[R]]
 ;
   %ab = shufflevector <8 x i8> %a, <8 x i8> poison, <4 x i32> <i32 3, i32 2, i32 1, i32 0>
@@ -332,11 +328,7 @@ define <8 x i8> @constantdiff2(<8 x i8> %a) {
 
 define <8 x half> @constantsplatf(<8 x half> %a) {
 ; CHECK-LABEL: @constantsplatf(
-; CHECK-NEXT:    [[AB:%.*]] = shufflevector <8 x half> [[A:%.*]], <8 x half> poison, <4 x i32> <i32 3, i32 2, i32 1, i32 0>
-; CHECK-NEXT:    [[AT:%.*]] = shufflevector <8 x half> [[A]], <8 x half> poison, <4 x i32> <i32 7, i32 6, i32 5, i32 4>
-; CHECK-NEXT:    [[ABT:%.*]] = fadd <4 x half> [[AT]], <half 0xH4900, half 0xH4900, half 0xH4900, half 0xH4900>
-; CHECK-NEXT:    [[ABB:%.*]] = fadd <4 x half> [[AB]], <half 0xH4900, half 0xH4900, half 0xH4900, half 0xH4900>
-; CHECK-NEXT:    [[R:%.*]] = shufflevector <4 x half> [[ABT]], <4 x half> [[ABB]], <8 x i32> <i32 7, i32 6, i32 5, i32 4, i32 3, i32 2, i32 1, i32 0>
+; CHECK-NEXT:    [[R:%.*]] = fadd <8 x half> [[A:%.*]], <half 0xH4900, half 0xH4900, half 0xH4900, half 0xH4900, half 0xH4900, half 0xH4900, half 0xH4900, half 0xH4900>
 ; CHECK-NEXT:    ret <8 x half> [[R]]
 ;
   %ab = shufflevector <8 x half> %a, <8 x half> poison, <4 x i32> <i32 3, i32 2, i32 1, i32 0>