[InstCombine] Support negation of scalable-vector splats

[frasercrmck]

Summary

This patch is an extension of D103421. It allows the InstCombiner to
generate the negated form of integer scalable-vector splats. It can
technically handle fixed-length vectors too but those are completely
covered by the preceding logic.

This enables extra combining opportunities for scalable vector types.

[RKSimon]

LGTM - cheers

llvm/lib/Transforms/InstCombine/InstructionCombining.cpp

for (unsigned i = 0, e = CV->getNumOperands(); i != e; ++i) {
         continue;
 
       if (!isa<ConstantInt>(Elt))
         return nullptr;
     }
     return ConstantExpr::getNeg(CV);
   }
 
+  // Negate integer vector splats.
+  if (auto *CV = dyn_cast<Constant>(V))
+    if (CV->getType()->isVectorTy() &&
+        CV->getType()->getScalarType()->isIntegerTy() && CV->getSplatValue())
+      return ConstantExpr::getNeg(CV);
+
   return nullptr;
 }
 
 static Value *foldOperationIntoSelectOperand(Instruction &I, Value *SO,
                                              InstCombiner::BuilderTy &Builder) {
   if (auto *Cast = dyn_cast<CastInst>(&I))
     return Builder.CreateCast(Cast->getOpcode(), SO, I.getType());
 

llvm/test/Transforms/InstCombine/sub.ll

 ; CHECK-LABEL: @test44vec(
 ; CHECK-NEXT:    [[SUB:%.*]] = add nsw <2 x i32> [[X:%.*]], <i32 -32768, i32 -32768>
 ; CHECK-NEXT:    ret <2 x i32> [[SUB]]
 ;
   %sub = sub nsw <2 x i32> %x, <i32 32768, i32 32768>
   ret <2 x i32> %sub
 }
 
-; FIXME: We're not giving this new 'add' a nsw flag as in the fixed-length case
-; above. We need to be able catch the splat with dyn_castNegVal.
 define <vscale x 2 x i32> @test44scalablevec(<vscale x 2 x i32> %x) {
 ; CHECK-LABEL: @test44scalablevec(
-; CHECK-NEXT:    [[SUB:%.*]] = add <vscale x 2 x i32> [[X:%.*]], shufflevector (<vscale x 2 x i32> insertelement (<vscale x 2 x i32> undef, i32 -32768, i32 0), <vscale x 2 x i32> undef, <vscale x 2 x i32> zeroinitializer)
+; CHECK-NEXT:    [[SUB:%.*]] = add nsw <vscale x 2 x i32> [[X:%.*]], shufflevector (<vscale x 2 x i32> insertelement (<vscale x 2 x i32> undef, i32 -32768, i32 0), <vscale x 2 x i32> undef, <vscale x 2 x i32> zeroinitializer)
 ; CHECK-NEXT:    ret <vscale x 2 x i32> [[SUB]]
 ;
   %sub = sub nsw <vscale x 2 x i32> %x, shufflevector (<vscale x 2 x i32> insertelement (<vscale x 2 x i32> undef, i32 32768, i32 0), <vscale x 2 x i32> undef, <vscale x 2 x i32> zeroinitializer)
   ret <vscale x 2 x i32> %sub
 }
 
 define <2 x i16> @test44vecminval(<2 x i16> %x) {
 ; CHECK-LABEL: @test44vecminval(