[TargetLowering] Remove APInt divisor argument from BuildExactSDIV (NFCI).

As requested in D50392, this is a minor refactor to BuildExactSDIV to stop taking the uniform constant APInt divisor and instead extract it locally.

I also cleanup the operands and valuetypes to better match BuildUDiv (and BuildSDIV in the near future).

llvm-svn: 339246