Moves APInt and corresponding convert to float method into the pnacl
translator. Renames APInt to BitcodeInt to be more clear that we aren't referring to LLVM's APInt. Rather, it is the representation in PNaCl bitcode.

BUG=None
R=stichnot@chromium.org

Review URL: https://codereview.chromium.org/1484003003 .