MIR Serialization: Serialize the register mask machine operands.

This commit implements serialization of the register mask machine
operands. This commit serializes only the call preserved register
masks that are defined by a target, it doesn't serialize arbitrary
register masks.

This commit also extends the TargetRegisterInfo class and TableGen so that
the users of TRI can get the list of all the call preserved register masks and
their names.

Reviewers: Duncan P. N. Exon Smith

Differential Revision: http://reviews.llvm.org/D10673


git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@240966 91177308-0d34-0410-b5e6-96231b3b80d8

include/llvm/Target/TargetRegisterInfo.h

@@ -469,6 +469,10 @@ public:
     return nullptr;
   }
 
+  /// Return all the call-preserved register masks defined for this target.
+  virtual ArrayRef<const uint32_t *> getRegMasks() const = 0;
+  virtual ArrayRef<const char *> getRegMaskNames() const = 0;
+
   /// getReservedRegs - Returns a bitset indexed by physical register number
   /// indicating if a register is a special register that has particular uses
   /// and should be considered unavailable at all times, e.g. SP, RA. This is