RegisterCoalescer: Shrink main range after shrinking subranges

If the subregister uses were dead, this would leave the main range
segment pointing to a deleted instruction.

Not sure if this should try to avoid shrinking if we know we don't
have dead components.

Author: arsenm

llvm/lib/CodeGen/RegisterCoalescer.cpp

@@ -2103,6 +2103,7 @@ bool RegisterCoalescer::joinCopy(MachineInstr *CopyMI, bool &Again) {
       LLVM_DEBUG(dbgs() << "Shrink LaneUses (Lane " << PrintLaneMask(S.LaneMask)
                         << ")\n");
       LIS->shrinkToUses(S, LI.reg());
+      ShrinkMainRange = true;
     }
     LI.removeEmptySubRanges();
   }

llvm/test/CodeGen/AMDGPU/coalesce-into-dead-subreg-copies.mir

@@ -0,0 +1,33 @@
+# NOTE: Assertions have been autogenerated by utils/update_mir_test_checks.py
+# RUN: llc -mtriple=amdgcn-amd-amdhsa -mcpu=gfx900 -verify-coalescing -run-pass=simple-register-coalescing -o - %s | FileCheck %s
+
+# Check that there's no  "Live segment doesn't end at a valid
+# instruction" failure after coalescing %0 into %2, which is
+# ultimately a pair of dead copies.
+
+---
+name: coalesce_into_dead_subreg_copy
+tracksRegLiveness: true
+machineFunctionInfo:
+  isEntryFunction: true
+  scratchRSrcReg:  '$sgpr0_sgpr1_sgpr2_sgpr3'
+  stackPtrOffsetReg: '$sgpr32'
+  occupancy:       8
+body:             |
+  ; CHECK-LABEL: name: coalesce_into_dead_subreg_copy
+  ; CHECK: bb.0:
+  ; CHECK-NEXT:   successors: %bb.1(0x80000000)
+  ; CHECK-NEXT: {{  $}}
+  ; CHECK-NEXT:   dead %2:sreg_64_xexec = S_LOAD_DWORDX2_IMM undef %1:sgpr_64, 24, 0 :: (dereferenceable invariant load (s64), addrspace 4)
+  ; CHECK-NEXT:   S_BRANCH %bb.1
+  ; CHECK-NEXT: {{  $}}
+  ; CHECK-NEXT: bb.1:
+  bb.0:
+    %0:sreg_64_xexec = S_LOAD_DWORDX2_IMM undef %1:sgpr_64, 24, 0 :: (dereferenceable invariant load (s64), addrspace 4)
+    undef %2.sub0:sreg_64 = COPY %0.sub0:sreg_64_xexec
+    %2.sub1:sreg_64 = COPY killed %0.sub1:sreg_64_xexec
+    S_BRANCH %bb.1
+
+  bb.1:
+
+...

llvm/test/CodeGen/AMDGPU/loop-live-out-copy-undef-subrange.ll

@@ -10,7 +10,7 @@ define <3 x float> @liveout_undef_subrange(<3 x float> %arg) {
 ; CHECK:       ; %bb.0: ; %bb
 ; CHECK-NEXT:    s_waitcnt vmcnt(0) expcnt(0) lgkmcnt(0)
 ; CHECK-NEXT:    v_add_f32_e32 v3, v2, v2
-; CHECK-NEXT:    v_add_f32_e32 v1, v1, v1
+; CHECK-NEXT:    ; kill: killed $vgpr1
 ; CHECK-NEXT:    v_add_f32_e32 v0, v0, v0
 ; CHECK-NEXT:  .LBB0_1: ; %bb1
 ; CHECK-NEXT:    ; =>This Loop Header: Depth=1