[msp430] Only support the 'm' inline assembly memory constraint. NFC.

[dsanders]

Summary

MSP430 doesn't seem to have 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.

[dsanders]

Ping 2...

I'm not sure that there is active development on MSP430 so I'm not sure who would review this. Looking through the commit logs for likely reviewers I can only see infrastructure changes this year..

[dsanders]

I had a quick chat with Eric Christopher at EuroLLVM about this patch and he suggested I go ahead and commit this one.

lib/Target/MSP430/MSP430ISelLowering.h

public:
 
     TargetLowering::ConstraintType
     getConstraintType(const std::string &Constraint) 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;
-    }
-
     /// isTruncateFree - Return true if it's free to truncate a value of type
     /// Ty1 to type Ty2. e.g. On msp430 it's free to truncate a i16 value in
     /// register R15W to i8 by referencing its sub-register R15B.
     bool isTruncateFree(Type *Ty1, Type *Ty2) const override;
     bool isTruncateFree(EVT VT1, EVT VT2) const override;
 
     /// isZExtFree - Return true if any actual instruction that defines a value
     /// of type Ty1 implicit zero-extends the value to Ty2 in the result