[NFC][CodeGen][X86][AArch64] div-rem pair reconstruction tests (PR42673)

As discussed in https://bugs.llvm.org/show_bug.cgi?id=42673
there is a TTI hook hasDivRemOp() that matters here.
While -div-rem-pairs will decompose 'rem' if that hook returns false,
nothing does the opposite transform.

We can't to this in InstCombine, because it does not currently
access TTI, and i'm not sure we should change that.

We may be able to teach DivRemPairs to do this, but this really is a
per-target perf optimization, and we seem to do the opposite transform
in backend if hasDivRemOp() returned false: https://godbolt.org/z/ttt4HZ
I think it makes sense to be consistent.

https://bugs.llvm.org/show_bug.cgi?id=42673

llvm-svn: 367034