Fix -Wconversion warnings for ResolvedFieldEntry

The recent change in JDK-8301996 added more -Wconversion warnings that need to be addressed.

Also:
rewriter.cpp: In member function 'void Rewriter::rewrite_field_reference(address, int, bool)':
src/hotspot/share/interpreter/rewriter.cpp:188:29: warning: conversion from 'int' to 'u2' {aka 'short unsigned int'} may change value [-Wconversion]
  188 | Bytes::put_native_u2(p, field_entry_index);
      | ^~~~~~~~~~~~~~~~~
src/hotspot/share/interpreter/rewriter.cpp:192:27: warning: conversion from 'int' to 'u2' {aka 'short unsigned int'} may change value [-Wconversion]
  192 | Bytes::put_Java_u2(p, pool_index);
      | ^~~~~~~~~~