RegisterCoalescer: Clear isSSA property

Fix some expensive checks verifier errors on MIR tests.

Author: arsenm

llvm/lib/CodeGen/RegisterCoalescer.cpp

@@ -387,6 +387,11 @@ namespace {
 
     void getAnalysisUsage(AnalysisUsage &AU) const override;
 
+    MachineFunctionProperties getClearedProperties() const override {
+      return MachineFunctionProperties().set(
+          MachineFunctionProperties::Property::IsSSA);
+    }
+
     void releaseMemory() override;
 
     /// This is the pass entry point.