The runtime helpers are given more consistent names:
  __Sz_<bitcode>_<type1>_<type2>

Missing helpers (various vector bitcasts) are implemented.

We'd prefer to avoid calling external library functions, e.g. in compiler-rt, but even llc uses these helpers for some bitcode on x86-32, so the alternative is to copy their implementation into Subzero's runtime.

BUG= none
R=mtrofin@chromium.org

Review URL: https://codereview.chromium.org/961413002