fix `@bitCast` when src/dest types have mismatched handle_is_ptr

* separate BitCast and BitCastGen instructions
 * closes #991
 * closes #1934
 * unrelated: fix typo in docs (thanks gamester for pointing it out)
0c5f897904Andrew Kelley committed on 2/22/2019, 1:49:27 PMparentcbce61a
6 files changedLine totals unavailable