Add IntLog2 and use it

Replace uses of floating point log2 when we want an integer result from
an integer operand.  This avoids concerns about accuracy of floating
point library functions.

gtests/CMakeLists.txt

@@ -47,6 +47,7 @@ if(BUILD_TESTING)
             # Test related source files
             ${CMAKE_CURRENT_SOURCE_DIR}/AST.FromFile.cpp
             ${CMAKE_CURRENT_SOURCE_DIR}/BuiltInResource.FromFile.cpp
+            ${CMAKE_CURRENT_SOURCE_DIR}/Common.cpp
             ${CMAKE_CURRENT_SOURCE_DIR}/Config.FromFile.cpp
             ${CMAKE_CURRENT_SOURCE_DIR}/HexFloat.cpp
             ${CMAKE_CURRENT_SOURCE_DIR}/Hlsl.FromFile.cpp