[AMDGPU] Fixed asan failure in SIFoldOperands

[rampitec]

Summary

Both tryFoldOMod() and tryFoldClamp() remove original instruction,
so the check MI.modifiesRegister() may use a deleted MI.

[arsenm]

LGTM

llvm/lib/Target/AMDGPU/SIFoldOperands.cpp

for (MachineBasicBlock *MBB : depth_first(&MF)) {
     MachineOperand *CurrentKnownM0Val = nullptr;
     for (I = MBB->begin(); I != MBB->end(); I = Next) {
       Next = std::next(I);
       MachineInstr &MI = *I;
 
       tryFoldInst(TII, &MI);
 
       if (!TII->isFoldableCopy(MI)) {
+        // Saw an unknown clobber of m0, so we no longer know what it is.
+        if (CurrentKnownM0Val && MI.modifiesRegister(AMDGPU::M0, TRI))
+          CurrentKnownM0Val = nullptr;
+
         // TODO: Omod might be OK if there is NSZ only on the source
         // instruction, and not the omod multiply.
         if (IsIEEEMode || (!HasNSZ && !MI.getFlag(MachineInstr::FmNsz)) ||
             !tryFoldOMod(MI))
           tryFoldClamp(MI);
 
-        // Saw an unknown clobber of m0, so we no longer know what it is.
-        if (CurrentKnownM0Val && MI.modifiesRegister(AMDGPU::M0, TRI))
-          CurrentKnownM0Val = nullptr;
         continue;
       }
 
       // Specially track simple redefs of m0 to the same value in a block, so we
       // can erase the later ones.
       if (MI.getOperand(0).getReg() == AMDGPU::M0) {
         MachineOperand &NewM0Val = MI.getOperand(1);
         if (CurrentKnownM0Val && CurrentKnownM0Val->isIdenticalTo(NewM0Val)) {