[GISel] Legalize G_FSUB to G_FADD + G_FNEG even if G_FNEG is illegal

[barannikov88]

Summary

G_FNEG used to be legalized to G_FSUB -0, x causing infinite loop.
This is no longer the case after D84287.

[barannikov88]

Any idea how to test it?

[arsenm]

LGTM. I don't think there's anything to test

llvm/lib/CodeGen/GlobalISel/LegalizerHelper.cpp

case TargetOpcode::G_FNEG: {
     return Legalized;
   }
   case TargetOpcode::G_FSUB:
   case TargetOpcode::G_STRICT_FSUB: {
     auto [Res, LHS, RHS] = MI.getFirst3Regs();
     LLT Ty = MRI.getType(Res);
 
     // Lower (G_FSUB LHS, RHS) to (G_FADD LHS, (G_FNEG RHS)).
-    // First, check if G_FNEG is marked as Lower. If so, we may
-    // end up with an infinite loop as G_FSUB is used to legalize G_FNEG.
-    if (LI.getAction({G_FNEG, {Ty}}).Action == Lower)
-      return UnableToLegalize;
     auto Neg = MIRBuilder.buildFNeg(Ty, RHS);
 
     if (MI.getOpcode() == TargetOpcode::G_STRICT_FSUB)
       MIRBuilder.buildStrictFAdd(Res, LHS, Neg, MI.getFlags());
     else
       MIRBuilder.buildFAdd(Res, LHS, Neg, MI.getFlags());
 
     MI.eraseFromParent();