[sil] Add a new helper method CondBranchInst::isConditionOperand.

I am doing some refactoring on BranchPropagatedUser that is requiring me to work
a lot more with arguments and I found a need for this to avoid trafficing in
operand indices.

As part of this I also needed to write the helper getConditionOperand() to get
the underlying operand. I refactored the already preset getCondition() to use
this helper instead since it was just semantically that method + get operand's
value.

Author: gottesmm

include/swift/SIL/SILInstruction.h

@@ -7213,7 +7213,10 @@ class CondBranchInst final
          ProfileCounter FalseBBCount, SILFunction &F);
 
 public:
-  SILValue getCondition() const { return getAllOperands()[ConditionIdx].get(); }
+  const Operand *getConditionOperand() const {
+    return &getAllOperands()[ConditionIdx];
+  }
+  SILValue getCondition() const { return getConditionOperand()->get(); }
   void setCondition(SILValue newCondition) {
     getAllOperands()[ConditionIdx].set(newCondition);
   }
@@ -7259,6 +7262,11 @@ class CondBranchInst final
     return getAllOperands().slice(NumFixedOpers + getNumTrueArgs());
   }
 
+  /// Returns true if \p op is mapped to the condition operand of the cond_br.
+  bool isConditionOperand(Operand *op) const {
+    return getConditionOperand() == op;
+  }
+
   bool isConditionOperandIndex(unsigned OpIndex) const {
     assert(OpIndex < getNumOperands() &&
            "OpIndex must be an index for an actual operand");