[Triton-MLIR][OPTIMIZER] Cleaned up swizzling (#869)

Swizzling is no longer implemented as a separate pass. It is instead
done in a specialized constructor of SharedEncodingAttr, and tested via
google tests instead of triton-opt + filecheck.

In the future we may want to implement it as a pass again once we have
an additional dialect between TritonGPU and LLVM.

unittest/Dialect/TritonGPU/CMakeLists.txt

@@ -0,0 +1,6 @@
+
+add_triton_ut(
+	NAME TestSwizzling
+	SRCS SwizzleTest.cpp
+	LIBS TritonGPUIR  ${dialect_libs} ${conversion_libs}
+)