Fix -Wconversion warnings in classfile code

Various -Wconversion warnings in classfile/* source files.

11 src/hotspot/share/classfile/classFileParser.cpp
8 src/hotspot/share/classfile/symbolTable.cpp
5 src/hotspot/share/classfile/bytecodeAssembler.cpp
3 src/hotspot/share/classfile/vmIntrinsics.cpp
3 src/hotspot/share/classfile/stringTable.cpp
3 src/hotspot/share/classfile/defaultMethods.cpp
3 src/hotspot/share/classfile/compactHashtable.cpp
3 src/hotspot/share/classfile/classLoader.cpp
2 src/hotspot/share/classfile/systemDictionaryShared.cpp
2 src/hotspot/share/classfile/klassFactory.cpp
2 src/hotspot/share/classfile/fieldLayoutBuilder.cpp
1 src/hotspot/share/classfile/loaderConstraints.cpp
1 src/hotspot/share/classfile/classLoaderExt.cpp
1 src/hotspot/share/classfile/altHashing.cpp