Reland r338431: "Add DebugCounters to DivRemPairs"

(Previously reverted in r338442)

I'm told that the breakage came from us using an x86 triple on configs
that didn't have x86 enabled. This is remedied by moving the
debugcounter test to an x86 directory (where there's also a
opt-bisect-isel.ll test for similar reasons).

I can't repro the reverse-iteration failure mentioned in the revert with
this patch, so I assume that a misconfiguration on my end is what caused
that.

Original commit message:

    Add DebugCounters to DivRemPairs

    For people who don't use DebugCounters, NFCI.

    Patch by Zhizhou Yang!

    Differential Revision: https://reviews.llvm.org/D50033

llvm-svn: 338653