[SPARC] Fix-forward llvm#154950 by returning true if SP::V8BAR is handled (llvm#162424)

Use-after-poison happens because after SP::V8BAR is handled, it erases
MI, which therefore should not be inspected by `ExpandPostRA::run()`.

This fixes a buildbot-reported issue from llvm#154950
(https://lab.llvm.org/buildbot/#/builders/24/builds/13433).

Author: thurstond

llvm/lib/Target/Sparc/SparcInstrInfo.cpp

@@ -668,6 +668,7 @@ bool SparcInstrInfo::expandPostRAPseudo(MachineInstr &MI) const {
              .addImm(-1);
     MIBundleBuilder(MBB, InstSTBAR, InstLDSTUB);
     MBB.erase(MI);
+    return true;
   }
   }
   return false;