[InitUndef] handleSubReg should skip artificial subregs.

When enabling subreg liveness tracking for AArch64, this pass fails
because it tries to get the register class for the artificial subreg
`sub_32_hi` of a 64-bit GPR. It tries to create an INIT_UNDEF instruction
for the top 32-bits of the 64-bit GPR, which are not directly addressable,
so getSubRegisterClass() returns a nullptr, crashing this pass.

It should instead just avoid trying to create the INIT_UNDEF instruction.

Author: sdesmalen-arm

llvm/lib/CodeGen/InitUndef.cpp

@@ -164,6 +164,15 @@ bool InitUndef::handleSubReg(MachineFunction &MF, MachineInstr &MI,
     TRI->getCoveringSubRegIndexes(*MRI, TargetRegClass, NeedDef,
                                   SubRegIndexNeedInsert);
 
+    // It's not possible to create the INIT_UNDEF when there is no register
+    // class associated for the subreg. This may happen for artificial subregs
+    // that are not directly addressable.
+    if (any_of(SubRegIndexNeedInsert,
+               [&TRI = TRI, &TargetRegClass](unsigned ind) -> bool {
+                 return !TRI->getSubRegisterClass(TargetRegClass, ind);
+               }))
+      continue;
+
     Register LatestReg = Reg;
     for (auto ind : SubRegIndexNeedInsert) {
       Changed = true;

llvm/test/CodeGen/AArch64/init-undef.mir

@@ -1,5 +1,6 @@
 # NOTE: Assertions have been autogenerated by utils/update_mir_test_checks.py UTC_ARGS: --version 5
-# RUN: llc -mtriple=aarch64-- -run-pass=init-undef -o - %s | FileCheck %s
+# RUN: llc -mtriple=aarch64-- -aarch64-enable-subreg-liveness-tracking=false -run-pass=init-undef -o - %s | FileCheck %s
+# RUN: llc -mtriple=aarch64-- -aarch64-enable-subreg-liveness-tracking=true -run-pass=init-undef -o - %s | FileCheck %s
 
 ---
 name:            test_stxp_undef