[AMDGPU] Check for num elts in SelectVOP3PMods

[piotr]

Summary

The rest of the code section assumes there are exactly two elements
in the vector (Lo, Hi), so add the check before entering the section.

[arsenm]

Does this solve a real problem?

[piotr]

In D133852#3789211, @arsenm wrote:

Does this solve a real problem?

Yes, the wrong path can be entered by one of the v_wmma instructions.

[arsenm]

In D133852#3789325, @piotr wrote:

In D133852#3789211, @arsenm wrote:

Does this solve a real problem?

Yes, the wrong path can be entered by one of the v_wmma instructions.

Should get a test then

[piotr]

Added test.

llvm/lib/Target/AMDGPU/AMDGPUISelDAGToDAG.cpp

bool AMDGPUDAGToDAGISel::SelectVOP3PMods(SDValue In, SDValue &Src,
   unsigned Mods = 0;
   Src = In;
 
   if (Src.getOpcode() == ISD::FNEG) {
     Mods ^= (SISrcMods::NEG | SISrcMods::NEG_HI);
     Src = Src.getOperand(0);
   }
 
-  if (Src.getOpcode() == ISD::BUILD_VECTOR &&
+  if (Src.getOpcode() == ISD::BUILD_VECTOR && Src.getNumOperands() == 2 &&
       (!IsDOT || !Subtarget->hasDOTOpSelHazard())) {
     unsigned VecMods = Mods;
 
     SDValue Lo = stripBitcast(Src.getOperand(0));
     SDValue Hi = stripBitcast(Src.getOperand(1));
 
     if (Lo.getOpcode() == ISD::FNEG) {
       Lo = stripBitcast(Lo.getOperand(0));