[AMDGPU] Prevent hang in SIFoldOperands

SI Folding REG_SEQUENCE can hang if RSUse is used in multiple MIs
Added repro mir testcase.

Author: choikwa

llvm/lib/Target/AMDGPU/SIFoldOperands.cpp

@@ -13,6 +13,7 @@
 #include "MCTargetDesc/AMDGPUMCTargetDesc.h"
 #include "SIMachineFunctionInfo.h"
 #include "llvm/ADT/DepthFirstIterator.h"
+#include "llvm/ADT/DenseSet.h"
 #include "llvm/CodeGen/MachineFunctionPass.h"
 #include "llvm/CodeGen/MachineOperand.h"
 
@@ -74,6 +75,7 @@ class SIFoldOperands : public MachineFunctionPass {
   const SIRegisterInfo *TRI;
   const GCNSubtarget *ST;
   const SIMachineFunctionInfo *MFI;
+  mutable DenseSet<MachineInstr *> SeenMI;
 
   bool frameIndexMayFold(const MachineInstr &UseMI, int OpNo,
                          const MachineOperand &OpToFold) const;
@@ -772,7 +774,6 @@ void SIFoldOperands::foldOperand(
   if (UseMI->isRegSequence()) {
     Register RegSeqDstReg = UseMI->getOperand(0).getReg();
     unsigned RegSeqDstSubReg = UseMI->getOperand(UseOpIdx + 1).getImm();
-
     for (auto &RSUse : make_early_inc_range(MRI->use_nodbg_operands(RegSeqDstReg))) {
       MachineInstr *RSUseMI = RSUse.getParent();
 
@@ -783,6 +784,10 @@ void SIFoldOperands::foldOperand(
       if (RSUse.getSubReg() != RegSeqDstSubReg)
         continue;
 
+      if (SeenMI.contains(RSUseMI))
+        continue;
+      SeenMI.insert(RSUseMI);
+
       foldOperand(OpToFold, RSUseMI, RSUseMI->getOperandNo(&RSUse), FoldList,
                   CopiesToReplace);
     }

llvm/test/CodeGen/AMDGPU/si-fold-reg-sequence.mir

@@ -0,0 +1,16 @@
+# RUN: llc -mtriple=amdgcn-amd-amdhsa -mcpu=gfx900 -run-pass=si-fold-operands -verify-machineinstrs -o - %s
+
+---
+name:            fold_reg_sequence
+body:             |
+  bb.0:
+    liveins: $vgpr0_vgpr1, $vgpr2
+    %33:sreg_32 = S_MOV_B32 0
+    %34:sreg_32 = S_MOV_B32 429
+    %35:sreg_64 = REG_SEQUENCE killed %34, %subreg.sub0, %33, %subreg.sub1
+    %49:vgpr_32 = V_MUL_HI_U32_e64 $vgpr2, %35.sub0, implicit $exec
+    %75:vgpr_32 = GLOBAL_LOAD_DWORD $vgpr0_vgpr1, 0, 0, implicit $exec :: (load (s32), addrspace 1)
+    %77:vgpr_32 = V_MUL_HI_U32_e64 %75, %35.sub0, implicit $exec
+    S_ENDPGM 0
+...
+