Runtime code in the oops directory stores 'int' values into 'u2' containers for ConstantPool and ConstMethod. Changing return types to u2 and using u2 as variables, avoids -Wconversion warnings in these source files. There's some fan out. To avoid infinite fan out, added some check_cast<u2> casts for types known to be u2 as specified in the JVMS.