GlobalISel: Add accessor to known bits to CombinerHelper

[arsenm]

Summary

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

[paquette]

This seems reasonable to me.

(Also yeah I feel like we need a better way to do target combines)

[arsenm]

a87ca9e4af5a794054a179688a1d73de0e97d81b

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

protected:
   GISelKnownBits *KB;
   MachineDominatorTree *MDT;
 
 public:
   CombinerHelper(GISelChangeObserver &Observer, MachineIRBuilder &B,
                  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;
 
   /// Replace a single register operand with a new register and inform the
   /// observer of the changes.
   void replaceRegOpWith(MachineRegisterInfo &MRI, MachineOperand &FromRegOp,
                         Register ToReg) const;