Index: lib/CodeGen/RegAllocBase.cpp
===================================================================
--- lib/CodeGen/RegAllocBase.cpp
+++ lib/CodeGen/RegAllocBase.cpp
@@ -144,7 +144,6 @@
         continue;
       }
       DEBUG(dbgs() << "queuing new interval: " << *SplitVirtReg << "\n");
-      assert(!SplitVirtReg->empty() && "expecting non-empty interval");
       assert(TargetRegisterInfo::isVirtualRegister(SplitVirtReg->reg) &&
              "expect split value in virtual register");
       enqueue(SplitVirtReg);
Index: test/CodeGen/AMDGPU/spill-empty-live-interval.mir
===================================================================
--- /dev/null
+++ test/CodeGen/AMDGPU/spill-empty-live-interval.mir
@@ -0,0 +1,49 @@
+# RUN: llc -mtriple=amdgcn-amd-amdhsa-opencl -verify-machineinstrs -stress-regalloc=1 -start-before=simple-register-coalescing -stop-after=greedy -o - %s | FileCheck %s
+# https://bugs.llvm.org/show_bug.cgi?id=33620
+
+--- |
+
+  define amdgpu_kernel void @expecting_non_empty_interval() #0 {
+    ret void
+  }
+
+  attributes #0 = { nounwind }
+
+...
+---
+# This would assert due to the empty live interval created for %vreg9
+# on the last S_NOP with an undef subreg use.
+
+# CHECK-LABEL: name: expecting_non_empty_interval
+
+# CHECK: undef %7.sub1 = V_MAC_F32_e32 0, undef %1, undef %7.sub1, implicit %exec
+# CHECK-NEXT: SI_SPILL_V64_SAVE %7, %stack.0, %private_rsrc_reg, %fp_reg, 0, implicit %exec :: (store 8 into %stack.0, align 4)
+# CHECK-NEXT: undef %5.sub1 = V_MOV_B32_e32 1786773504, implicit %exec
+# CHECK-NEXT: dead %2 = V_MUL_F32_e32 0, %5.sub1, implicit %exec
+
+# CHECK: S_NOP 0, implicit %6.sub1
+# CHECK-NEXT: %8 = SI_SPILL_V64_RESTORE %stack.0, %private_rsrc_reg, %fp_reg, 0, implicit %exec :: (load 8 from %stack.0, align 4)
+# CHECK-NEXT: S_NOP 0, implicit %8.sub1
+# CHECK-NEXT: S_NOP 0, implicit undef %9.sub0
+
+name: expecting_non_empty_interval
+tracksRegLiveness: true
+registers:
+  - { id: 0, class: vreg_64, preferred-register: '' }
+  - { id: 1, class: vgpr_32, preferred-register: '' }
+  - { id: 2, class: vgpr_32, preferred-register: '' }
+  - { id: 3, class: vreg_64, preferred-register: '' }
+body:             |
+  bb.0:
+    successors: %bb.1
+    undef %0.sub1 = V_MAC_F32_e32 0, undef %1, undef %0.sub1, implicit %exec
+    undef %3.sub1 = V_MOV_B32_e32 1786773504, implicit %exec
+    dead %2 = V_MUL_F32_e32 0, %3.sub1, implicit %exec
+
+  bb.1:
+    S_NOP 0, implicit %3.sub1
+    S_NOP 0, implicit %0.sub1
+    S_NOP 0, implicit undef %0.sub0
+    S_ENDPGM
+
+...