Expand i8 selects into control flow instead of 16-bit conditional

moves. This avoids the need to promote the operands (or implicitly
extend them, a partial register update condition), and can reduce
i8 register pressure. This substantially speeds up code such as
write_hex in lib/Support/raw_ostream.cpp.

subclass-coalesce.ll is too trivial and no longer tests what it was
originally intended to test.

llvm-svn: 80184