[InstCombine] Fix a cycle when folding fneg(select) with scalable vector types

The two folding operations are causing a cycle for the following case with
scalable vector types:

define <vscale x 2 x double> @test_fneg_select_abs(<vscale x 2 x i1> %cond, <vscale x 2 x double> %b) {
  %1 = select <vscale x 2 x i1> %cond, <vscale x 2 x double> zeroinitializer, <vscale x 2 x double> %b
  %2 = fneg fast <vscale x 2 x double> %1
  ret <vscale x 2 x double> %2
}

1) fold fneg:  -(Cond ? C : Y) -> Cond ? -C : -Y

2) fold select: (Cond ? -X : -Y) -> -(Cond ? X : Y)

1) results in the following since '<vscale x 2 x double> zeroinitializer' passes
the check for the immediate constant:

%.neg = fneg fast <vscale x 2 x double> zeroinitializer
%b.neg = fneg fast <vscale x 2 x double> %b
%1 = select fast <vscale x 2 x i1> %cond, <vscale x 2 x double> %.neg, <vscale x 2 x double> %b.neg

and so we end up going back and forth between 1) and 2).

Don't perform 1) if we are dealing with scalable vector constants as there is no
immediate representation for a negative scalable zeroinitializer.

Author: ssijaric-nv

llvm/lib/Transforms/InstCombine/InstCombineAddSub.cpp

@@ -2878,7 +2878,8 @@ Instruction *InstCombinerImpl::visitFNeg(UnaryOperator &I) {
 
     // -(Cond ? X : C) --> Cond ? -X : -C
     // -(Cond ? C : Y) --> Cond ? -C : -Y
-    if (match(X, m_ImmConstant()) || match(Y, m_ImmConstant())) {
+    if ((match(X, m_ImmConstant()) && !isa<ScalableVectorType>(X->getType())) ||
+        (match(Y, m_ImmConstant()) && !isa<ScalableVectorType>(Y->getType()))) {
       Value *NegX = Builder.CreateFNegFMF(X, &I, X->getName() + ".neg");
       Value *NegY = Builder.CreateFNegFMF(Y, &I, Y->getName() + ".neg");
       SelectInst *NewSel = SelectInst::Create(Cond, NegX, NegY);

llvm/test/Transforms/InstCombine/fneg.ll

@@ -1109,4 +1109,16 @@ define float @test_fneg_select_maxnum(float %x) {
   ret float %neg
 }
 
+; Check that there's no infinite loop.
+define <vscale x 2 x double> @test_fneg_select_svec(<vscale x 2 x i1> %cond, <vscale x 2 x double> %b) {
+; CHECK-LABEL: @test_fneg_select_svec(
+; CHECK-NEXT:    [[TMP1:%.*]] = select <vscale x 2 x i1> [[COND:%.*]], <vscale x 2 x double> zeroinitializer, <vscale x 2 x double> [[B:%.*]]
+; CHECK-NEXT:    [[TMP2:%.*]] = fneg fast <vscale x 2 x double> [[TMP1]]
+; CHECK-NEXT:    ret <vscale x 2 x double> [[TMP2]]
+;
+  %1 = select <vscale x 2 x i1> %cond, <vscale x 2 x double> zeroinitializer, <vscale x 2 x double> %b
+  %2 = fneg fast <vscale x 2 x double> %1
+  ret <vscale x 2 x double> %2
+}
+
 !0 = !{}