GlobalISel: Remove unsigned variant of SrcOp

Force using Register.

One downside is the generated register enums require explicit
conversion.

llvm-svn: 364194
diff --git a/llvm/lib/Target/Mips/MipsMachineFunction.h b/llvm/lib/Target/Mips/MipsMachineFunction.h
index d9d53c8..aaa1e0e 100644
--- a/llvm/lib/Target/Mips/MipsMachineFunction.h
+++ b/llvm/lib/Target/Mips/MipsMachineFunction.h
@@ -32,8 +32,8 @@
   void setSRetReturnReg(unsigned Reg) { SRetReturnReg = Reg; }
 
   bool globalBaseRegSet() const;
-  unsigned getGlobalBaseReg();
-  unsigned getGlobalBaseRegForGlobalISel();
+  Register getGlobalBaseReg();
+  Register getGlobalBaseRegForGlobalISel();
 
   // Insert instructions to initialize the global base register in the
   // first MBB of the function.