Adding a new intrinsic function

Adding a new intrinsic/built-in function that is accessible from C/C++ requires four steps:

1. Add a definition for the function to NyuziToolchain/tools/clang/include/clang/Basic/BuiltinsNyuzi.def, for example:

   BUILTIN(__builtin_nyuzi_read_control_reg, "ii", "n")

2. Open NyuziToolchain/tools/clang/lib/CodeGen/CGBuiltin.cpp and look at the function CodeGenFunction::EmitNyuziBuiltinExpr. Add an entry to the switch statement:

   switch (BuiltinID) { ... case Nyuzi::BI__builtin_nyuzi_read_control_reg:

3. Edit NyuziToolchain/include/llvm/IR/IntrinsicsNyuzi.td and add a definition of the intrinsic to the LLVM backend

   def int_nyuzi_read_control_reg : Intrinsic<[llvm_i32_ty], [llvm_i32_ty], [], "llvm.nyuzi.__builtin_nyuzi_read_control_reg">;

4. Add a pattern to the instruction table NyuziToolchain/lib/Target/Nyuzi/NyuziInstrInfo.td

   def READ_CONTROL_REG : FormatMInst< (outs GPR32:$dest), (ins i32imm:$cr), "getcr $dest, $cr", [(set i32:$dest, (int_nyuzi_read_control_reg imm:$cr))], FmtM_ControlReg, 1>