[InstCombine] allow vector splats for add+and with high-mask

There might be a better way to specify the pre-conditions,
but this is hopefully clearer than the way it was written:
https://rise4fun.com/Alive/Jhk3

  Pre: C2 < 0 && isShiftedMask(C2) && (C1 == C1 & C2)
  %a = and %x, C2
  %r = add %a, C1
  =>
  %a2 = add %x, C1
  %r = and %a2, C2

llvm/lib/Transforms/InstCombine/InstCombineAddSub.cpp

@@ -958,6 +958,15 @@ Instruction *InstCombinerImpl::foldAddWithConstant(BinaryOperator &Add) {
     }
   }
 
+  // If all bits affected by the add are included in a high-bit-mask, do the
+  // add before the mask op:
+  // (X & 0xFF00) + xx00 --> (X + xx00) & 0xFF00
+  if (match(Op0, m_OneUse(m_And(m_Value(X), m_APInt(C2)))) &&
+      C2->isNegative() && C2->isShiftedMask() && *C == (*C & *C2)) {
+    Value *NewAdd = Builder.CreateAdd(X, ConstantInt::get(Ty, *C));
+    return BinaryOperator::CreateAnd(NewAdd, ConstantInt::get(Ty, *C2));
+  }
+
   return nullptr;
 }
 
@@ -1370,34 +1379,6 @@ Instruction *InstCombinerImpl::visitAdd(BinaryOperator &I) {
   if (haveNoCommonBitsSet(LHS, RHS, DL, &AC, &I, &DT))
     return BinaryOperator::CreateOr(LHS, RHS);
 
-  // FIXME: We already did a check for ConstantInt RHS above this.
-  // FIXME: Is this pattern covered by another fold? No regression tests fail on
-  // removal.
-  if (ConstantInt *CRHS = dyn_cast<ConstantInt>(RHS)) {
-    // (X & FF00) + xx00  -> (X+xx00) & FF00
-    Value *X;
-    ConstantInt *C2;
-    if (LHS->hasOneUse() &&
-        match(LHS, m_And(m_Value(X), m_ConstantInt(C2))) &&
-        CRHS->getValue() == (CRHS->getValue() & C2->getValue())) {
-      // See if all bits from the first bit set in the Add RHS up are included
-      // in the mask.  First, get the rightmost bit.
-      const APInt &AddRHSV = CRHS->getValue();
-
-      // Form a mask of all bits from the lowest bit added through the top.
-      APInt AddRHSHighBits(~((AddRHSV & -AddRHSV)-1));
-
-      // See if the and mask includes all of these bits.
-      APInt AddRHSHighBitsAnd(AddRHSHighBits & C2->getValue());
-
-      if (AddRHSHighBits == AddRHSHighBitsAnd) {
-        // Okay, the xform is safe.  Insert the new add pronto.
-        Value *NewAdd = Builder.CreateAdd(X, CRHS, LHS->getName());
-        return BinaryOperator::CreateAnd(NewAdd, C2);
-      }
-    }
-  }
-
   // add (select X 0 (sub n A)) A  -->  select X A n
   {
     SelectInst *SI = dyn_cast<SelectInst>(LHS);

llvm/test/Transforms/InstCombine/add.ll

@@ -740,8 +740,8 @@ define i8 @test34(i8 %A) {
 
 define i8 @masked_add(i8 %x) {
 ; CHECK-LABEL: @masked_add(
-; CHECK-NEXT:    [[AND1:%.*]] = add i8 [[X:%.*]], 96
-; CHECK-NEXT:    [[R:%.*]] = and i8 [[AND1]], -16
+; CHECK-NEXT:    [[TMP1:%.*]] = add i8 [[X:%.*]], 96
+; CHECK-NEXT:    [[R:%.*]] = and i8 [[TMP1]], -16
 ; CHECK-NEXT:    ret i8 [[R]]
 ;
   %and = and i8 %x, 240 ; 0xf0
@@ -751,8 +751,8 @@ define i8 @masked_add(i8 %x) {
 
 define <2 x i8> @masked_add_splat(<2 x i8> %x) {
 ; CHECK-LABEL: @masked_add_splat(
-; CHECK-NEXT:    [[AND:%.*]] = and <2 x i8> [[X:%.*]], <i8 -64, i8 -64>
-; CHECK-NEXT:    [[R:%.*]] = add <2 x i8> [[AND]], <i8 64, i8 64>
+; CHECK-NEXT:    [[TMP1:%.*]] = add <2 x i8> [[X:%.*]], <i8 64, i8 64>
+; CHECK-NEXT:    [[R:%.*]] = and <2 x i8> [[TMP1]], <i8 -64, i8 -64>
 ; CHECK-NEXT:    ret <2 x i8> [[R]]
 ;
   %and = and <2 x i8> %x, <i8 192, i8 192> ; 0xc0