In Subzero, we only need to be able to convert 64 bit constants in
bitcode files to the corresponding Ice integer or floating type. This
CL extracts the minimal implementation needed for Subzero. The intent
of this change is to remove loading unnecessary LLVM code into
(minimal) llvm2ice.

BUG=None
R=stichnot@chromium.org

Review URL: https://codereview.chromium.org/797323002