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

[dsanders]

Summary

SPARC doesn't seem to support any additional constraints. Therefore remove
the target hook.

No functional change intended. Depends on D8173.

[dsanders]

Ping.

The main thing I'm looking for is for is for someone with knowledge of the target to say whether they're aware of any supported constraints that this patch doesn't cover.

[venkatra]

LGTM.

Thanks,
Venkat

lib/Target/Sparc/SparcISelLowering.h

void LowerAsmOperandForConstraint(SDValue Op,
                                       std::string &Constraint,
                                       std::vector<SDValue> &Ops,
                                       SelectionDAG &DAG) const override;
     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;
-    }
-
     bool isOffsetFoldingLegal(const GlobalAddressSDNode *GA) const override;
     MVT getScalarShiftAmountTy(EVT LHSTy) const override { return MVT::i32; }
 
     /// getSetCCResultType - Return the ISD::SETCC ValueType
     EVT getSetCCResultType(LLVMContext &Context, EVT VT) const override;
 
     SDValue
       LowerFormalArguments(SDValue Chain,