[X86] Tune bypassing of slow division for Intel CPUs

64-bit integer division in Intel CPUs is extremely slow, much slower
than 32-bit division. On the other hand, 8-bit and 16-bit divisions
aren't any faster. The only important exception is Atom where DIV8
is fastest. Because of that, the patch
1) Enables bypassing of 64-bit division for Atom, Silvermont and
   all big cores.
2) Modifies 64-bit bypassing to use 32-bit division instead of
   16-bit one. This doesn't make the shorter division slower but
   increases chances of taking it. Moreover, it's much more likely
   to prove at compile-time that a value fits 32 bits and doesn't
   require a run-time check (e.g. zext i32 to i64).

Differential Revision: https://reviews.llvm.org/D28196

llvm-svn: 291800