[CodeGen] Enhance createFrom for sub-reg aware cloning

Instead of just cloning the virtual register, this
function now creates a new virtual register derived
from a subregister class of the original value.

llvm/include/llvm/CodeGen/LiveRangeEdit.h

diff --git a/llvm/include/llvm/CodeGen/LiveRangeEdit.h b/llvm/include/llvm/CodeGen/LiveRangeEdit.h
index d0ed3ff..4db5e80 100644
--- a/llvm/include/llvm/CodeGen/LiveRangeEdit.h
+++ b/llvm/include/llvm/CodeGen/LiveRangeEdit.h
@@ -154,8 +154,11 @@
 
   ArrayRef<Register> regs() const { return ArrayRef(NewRegs).slice(FirstNew); }
 
-  /// createFrom - Create a new virtual register based on OldReg.
-  Register createFrom(Register OldReg);
+  /// 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);
 
   /// 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 6fe1170..d8a2ad4 100644
--- a/llvm/lib/CodeGen/LiveRangeEdit.cpp
+++ b/llvm/lib/CodeGen/LiveRangeEdit.cpp
@@ -52,8 +52,16 @@
   return LI;
 }
 
-Register LiveRangeEdit::createFrom(Register OldReg) {
+Register LiveRangeEdit::createFrom(Register OldReg,
+                                   const TargetRegisterClass *RC) {
   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 (VRM) {
     VRM->setIsSplitFromReg(VReg, VRM->getOriginal(OldReg));
   }