[BypassSlowDivision] Improve our handling of divisions by constants

(This reapplies r314253.  r314253 was reverted on r314482 because of a
correctness regression on P100, but that regression was identified to be
something else.)

Summary:
Don't bail out on constant divisors for divisions that can be narrowed without
introducing control flow .  This gives us a 32 bit multiply instead of an
emulated 64 bit multiply in the generated PTX assembly.

Reviewers: jlebar

Subscribers: jholewinski, mcrosier, llvm-commits

Differential Revision: https://reviews.llvm.org/D38265

llvm-svn: 319677