RegisterCoalescer: Don't skip resolveConflicts for SubRegJoin

[arsenm]

Summary

Fixes second "Couldn't join subrange!" error in bug 39542

[arsenm]

However, this variant still fails:

body: |

bb.0:
  successors: %bb.1
  undef %0.sub1:sreg_64_xexec = IMPLICIT_DEF

bb.1:
  %0.sub0:sreg_64_xexec = S_MOV_B32 0
  %1:sreg_64 = COPY %0:sreg_64_xexec
  dead %0.sub1:sreg_64_xexec = COPY %0.sub0:sreg_64_xexec
  S_ENDPGM implicit %1.sub1:sreg_64

lib/CodeGen/RegisterCoalescer.cpp

       continue;
     LLVM_DEBUG(dbgs() << "\t\tconflict at " << printReg(Reg) << ':' << i << '@'
                       << LR.getValNumInfo(i)->def << '\n');
-    if (SubRangeJoin)
-      return false;
 
     ++NumLaneConflicts;
     assert(V.OtherVNI && "Inconsistent conflict resolution.");

test/CodeGen/AMDGPU/pytorch-cannot-join-subrange2.mir

+# NOTE: Assertions have been autogenerated by utils/update_mir_test_checks.py
+# RUN: llc -mtriple=amdgcn-amd-amdhsa -verify-coalescing -run-pass=simple-register-coalescing -verify-machineinstrs -o - %s | FileCheck %s
+---
+name:            couldnt_join_subrange
+tracksRegLiveness: true
+body:             |
+  bb.0:
+    ; CHECK-LABEL: name: couldnt_join_subrange
+    ; CHECK: undef %0.sub0:sreg_64 = S_MOV_B32 0
+    ; CHECK: %0.sub1:sreg_64 = COPY %0.sub0
+    ; CHECK: S_ENDPGM implicit %0.sub1
+    undef %0.sub0:sreg_64 = S_MOV_B32 0
+    %1:sreg_64 = COPY %0:sreg_64
+    %0.sub1:sreg_64 = COPY %0.sub0:sreg_64
+    S_ENDPGM implicit %1.sub1:sreg_64
+
+...