[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.

unittest/Conversion/TritonGPUToLLVM/CMakeLists.txt

@@ -1,5 +1,5 @@
 add_triton_ut(
-	NAME TritonGPUToLLVMTests
-	SRCS TritonGPUToLLVMTests.cpp
+	NAME PtxAsmFormatTest
+	SRCS PtxAsmFormatTest.cpp
 	LIBS TritonGPUToLLVM
 )