[LoopStrengthReduce] Don't use a DenseSet<int64_t> when we might add any valid int64_t to the set.

Summary:
SmallSetVector uses DenseSet, but that means we need to reserve some
values for the empty and tombstone keys.

It seems to me we should have a general way to let us store full-range
ints inside of DenseSets, and furthermore that we probably shouldn't
silently let you add ints into DenseSets without explicitly promising
that they're in range.  But that's a battle for another day; for now,
just fix this code, since we currently do something Very Bad when
compiling ffmpeg.

Fixes PR30914.

Reviewers: jeremyhu

Subscribers: llvm-commits, mzolotukhin

Differential Revision: https://reviews.llvm.org/D26323

llvm-svn: 286038