[BACKEND] Add C++ tests for PTXFormat and some tiny refinement (#109)

This PR does

1. Add some C++ tests for `PTXFormat`
2. Enhance the functionality of `PTXFormat`, make a `PTXInstr` instance
can be called multiple times similar as a C function.

lib/Conversion/TritonGPUToLLVM/CMakeLists.txt

@@ -1,6 +1,6 @@
 add_mlir_conversion_library(TritonGPUToLLVM
     TritonGPUToLLVM.cpp
-        PtxAsmFormat.cpp
+    PtxAsmFormat.cpp
 
     ADDITIONAL_HEADER_DIRS
     ${PROJECT_SOURCE_DIR}/include/triton/Conversion/TritonGPUToLLVM