[MachineSinking] Conservatively clear kill flags after coalescing.

This solves the problem of having a kill flag inside a loop
with a definition of the register prior to the loop:

%vreg368<def> ...

Inside loop:

        %vreg520<def> = COPY %vreg368
        %vreg568<def,tied1> = add %vreg341<tied0>, %vreg520<kill>

=> was coalesced into =>

        %vreg568<def,tied1> = add %vreg341<tied0>, %vreg368<kill>

MachineVerifier then complained:
*** Bad machine code: Virtual register killed in block, but needed live out. ***

The kill flag for %vreg368 is incorrect, and is cleared by this patch.

This is similar to the clearing done at the end of
MachineSinking::SinkInstruction().

Patch provided by Jonas Paulsson.

Reviewed by Quentin Colombet and Juergen Ributzka.

llvm-svn: 217427

llvm/lib/CodeGen/MachineSink.cpp

@@ -157,6 +157,11 @@ bool MachineSinking::PerformTrivialForwardCoalescing(MachineInstr *MI,
   DEBUG(dbgs() << "*** to: " << *MI);
   MRI->replaceRegWith(DstReg, SrcReg);
   MI->eraseFromParent();
+
+  // Conservatively, clear any kill flags, since it's possible that they are no
+  // longer correct.
+  MRI->clearKillFlags(SrcReg);
+
   ++NumCoalesces;
   return true;
 }