GlobalISel: Add accessor to known bits to CombinerHelper

I need to pass known bits to a target combine matcher (which for some
reason aren't methods in a subclass of CombinerHelper?)

Author: arsenm

llvm/include/llvm/CodeGen/GlobalISel/CombinerHelper.h

@@ -62,6 +62,10 @@ class CombinerHelper {
                  GISelKnownBits *KB = nullptr,
                  MachineDominatorTree *MDT = nullptr);
 
+  GISelKnownBits *getKnownBits() const {
+    return KB;
+  }
+
   /// MachineRegisterInfo::replaceRegWith() and inform the observer of the changes
   void replaceRegWith(MachineRegisterInfo &MRI, Register FromReg, Register ToReg) const;