Use one loop to collect subregs.

Author: vpykhtin

llvm/lib/Target/AMDGPU/GCNRewritePartialRegUses.cpp

@@ -386,23 +386,22 @@ void GCNRewritePartialRegUsesImpl::updateLiveIntervals(
 }
 
 bool GCNRewritePartialRegUsesImpl::rewriteReg(Register Reg) const {
-  auto Range = MRI->reg_nodbg_operands(Reg);
-  if (Range.empty() || any_of(Range, [](MachineOperand &MO) {
-        return MO.getSubReg() == AMDGPU::NoSubRegister; // Whole reg used. [1]
-      }))
-    return false;
-
-  auto *RC = MRI->getRegClass(Reg);
-  LLVM_DEBUG(dbgs() << "Try to rewrite partial reg " << printReg(Reg, TRI)
-                    << ':' << TRI->getRegClassName(RC) << '\n');
 
   // Collect used subregs.
   SubRegMap SubRegs;
   for (MachineOperand &MO : MRI->reg_nodbg_operands(Reg)) {
-    assert(MO.getSubReg() != AMDGPU::NoSubRegister); // Due to [1].
+    if (MO.getSubReg() == AMDGPU::NoSubRegister)
+      return false; // Whole reg used.
     SubRegs.try_emplace(MO.getSubReg());
   }
 
+  if (SubRegs.empty())
+    return false;
+
+  auto *RC = MRI->getRegClass(Reg);
+  LLVM_DEBUG(dbgs() << "Try to rewrite partial reg " << printReg(Reg, TRI)
+                    << ':' << TRI->getRegClassName(RC) << '\n');
+
   auto *NewRC = getMinSizeReg(RC, SubRegs);
   if (!NewRC) {
     LLVM_DEBUG(dbgs() << "  No improvement achieved\n");