[xcore] Only support the 'm' inline assembly memory constraint. NFC.

[dsanders]

Summary

XCore doesn't seem to have any additional constraints. Therefore remove
the target hook.

No functional change intended. Depends on D8173.

[dsanders]

Ping...

[• friedgold]

Sorry, I missed this first try around. Change looks good to me.

[dsanders]

Thanks. Committed in r237442.

lib/Target/XCore/XCoreISelLowering.h

private:
     SDValue LowerATOMIC_STORE(SDValue Op, SelectionDAG &DAG) const;
 
     // Inline asm support
     std::pair<unsigned, const TargetRegisterClass *>
     getRegForInlineAsmConstraint(const TargetRegisterInfo *TRI,
                                  const std::string &Constraint,
                                  MVT VT) const override;
 
-    unsigned getInlineAsmMemConstraint(
-        const std::string &ConstraintCode) const override {
-      // FIXME: Map different constraints differently.
-      return InlineAsm::Constraint_m;
-    }
-
     // Expand specifics
     SDValue TryExpandADDWithMul(SDNode *Op, SelectionDAG &DAG) const;
     SDValue ExpandADDSUB(SDNode *Op, SelectionDAG &DAG) const;
 
     SDValue PerformDAGCombine(SDNode *N, DAGCombinerInfo &DCI) const override;
 
     void computeKnownBitsForTargetNode(const SDValue Op,
                                        APInt &KnownZero,