[GlobalISel] Remove duplicates from possible mappings

[ehjogab]

Summary

Some targets will in getInstrAlternativeMappings() will return a list
that contains the mapping returned by getInstrMapping(), thereby causing
duplicates. This patch removes such duplicates, which reduces the amount
of work that needs to be done when selecting register banks.

[ehjogab]

Reduce number of necessary checks.

llvm/include/llvm/CodeGen/GlobalISel/RegisterBankInfo.h

public:
     /// at the index i.
     InstructionMapping(unsigned ID, unsigned Cost,
                        const ValueMapping *OperandsMapping,
                        unsigned NumOperands)
         : ID(ID), Cost(Cost), OperandsMapping(OperandsMapping),
           NumOperands(NumOperands) {
     }
 
+    /// Check if this mapping has the same \p OperandsMapping as another \p
+    /// Mapping.
+    bool isCompatibleMapping(const InstructionMapping &Mapping) const {
+      return this->OperandsMapping == Mapping.OperandsMapping;
+    }
+
     /// Default constructor.
     /// Use this constructor to express that the mapping is invalid.
     InstructionMapping() = default;
 
     /// Get the cost.
     unsigned getCost() const { return Cost; }
 
     /// Get the ID.

llvm/lib/CodeGen/GlobalISel/RegisterBankInfo.cpp

RegisterBankInfo::getInstrPossibleMappings(const MachineInstr &MI) const {
   const auto &Mapping = getInstrMapping(MI);
   if (Mapping.isValid()) {
     // Put the default mapping first.
     PossibleMappings.push_back(&Mapping);
   }
 
   // Then the alternative mapping, if any.
   InstructionMappings AltMappings = getInstrAlternativeMappings(MI);
-  for (const InstructionMapping *AltMapping : AltMappings)
+  for (const InstructionMapping *AltMapping : AltMappings) {
+    // Check that we don't already have such a mapping.
+    if (AltMapping->isCompatibleMapping(Mapping)) {
+      continue;
+    }
     PossibleMappings.push_back(AltMapping);
+  }
 #ifndef NDEBUG
   for (const InstructionMapping *Mapping : PossibleMappings)
     assert(Mapping->verify(MI) && "Mapping is invalid");
 #endif
   return PossibleMappings;
 }
 
 RegisterBankInfo::InstructionMappings