pass in subregidx rather than a regclass

llvm/include/llvm/CodeGen/LiveRangeEdit.h

diff --git a/llvm/include/llvm/CodeGen/LiveRangeEdit.h b/llvm/include/llvm/CodeGen/LiveRangeEdit.h
index c0c16f1..309dd57 100644
--- a/llvm/include/llvm/CodeGen/LiveRangeEdit.h
+++ b/llvm/include/llvm/CodeGen/LiveRangeEdit.h
@@ -155,11 +155,11 @@
 
   ArrayRef<Register> regs() const { return ArrayRef(NewRegs).slice(FirstNew); }
 
-  /// createFrom - Create a new virtual register based on OldReg. If \p RC is
-  /// non-null, constrain the register class of the new vreg. Partial reloads
-  /// of tuple regclasses will feed-in a subregclass derived from the regclass
-  /// of OldReg.
-  Register createFrom(Register OldReg, const TargetRegisterClass *RC = nullptr);
+  /// createFrom - Create a new virtual register based on OldReg. If \p
+  /// SubRegIdx is non-null, constrain the register class of the new vreg.
+  /// Partial reloads of tuple regclasses will feed-in a subregidx derived from
+  /// OldReg.
+  Register createFrom(Register OldReg, const unsigned SubRegIdx = {});
 
   /// create - Create a new register with the same class and original slot as
   /// parent.

llvm/lib/CodeGen/LiveRangeEdit.cpp

diff --git a/llvm/lib/CodeGen/LiveRangeEdit.cpp b/llvm/lib/CodeGen/LiveRangeEdit.cpp
index 9e0c6c8..682d492 100644
--- a/llvm/lib/CodeGen/LiveRangeEdit.cpp
+++ b/llvm/lib/CodeGen/LiveRangeEdit.cpp
@@ -52,15 +52,20 @@
   return LI;
 }
 
-Register LiveRangeEdit::createFrom(Register OldReg,
-                                   const TargetRegisterClass *RC) {
+Register LiveRangeEdit::createFrom(Register OldReg, const unsigned SubRegIdx) {
   Register VReg = MRI.cloneVirtualRegister(OldReg);
 
-  // If RC is given, set it now. This is needed for cloning a new VReg with a
-  // smaller RC from its original virtual register, mainly used for subreg
-  // reload. The cloning is done first to ensure the vreg flags are preserved.
-  if (RC)
-    MRI.setRegClass(VReg, RC);
+  // If SubRegIdx is given, set it now. This is needed for cloning a new VReg
+  // with a smaller RC from its original virtual register, mainly used for
+  // subreg reload. The cloning is done first to ensure the vreg flags are
+  // preserved.
+  if (SubRegIdx) {
+    const TargetRegisterInfo &TRI = *MRI.getTargetRegisterInfo();
+    const TargetRegisterClass *RC =
+        TRI.getSubRegisterClass(MRI.getRegClass(OldReg), SubRegIdx);
+    if (RC)
+      MRI.setRegClass(VReg, RC);
+  }
 
   if (VRM) {
     VRM->setIsSplitFromReg(VReg, VRM->getOriginal(OldReg));