TargetMachine: Indicate whether machine verifier passes.

This adds a callback to the LLVMTargetMachine that lets target indicate
that they do not pass the machine verifier checks in all cases yet.

This is intended to be a temporary measure while the targets are fixed
allowing us to enable the machine verifier by default with
EXPENSIVE_CHECKS enabled!

Differential Revision: https://reviews.llvm.org/D33696

llvm-svn: 304320
diff --git a/llvm/lib/Target/PowerPC/PPCTargetMachine.h b/llvm/lib/Target/PowerPC/PPCTargetMachine.h
index f283835..b8f5a20 100644
--- a/llvm/lib/Target/PowerPC/PPCTargetMachine.h
+++ b/llvm/lib/Target/PowerPC/PPCTargetMachine.h
@@ -55,6 +55,10 @@
     const Triple &TT = getTargetTriple();
     return (TT.getArch() == Triple::ppc64 || TT.getArch() == Triple::ppc64le);
   };
+
+  bool isMachineVerifierClean() const override {
+    return false;
+  }
 };
 
 /// PowerPC 32-bit target machine.