Factor out common parts of LVI and Float2Int into ConstantRange [NFCI]

This just extracts out the transfer rules for constant ranges into a single shared point. As it happens, neither bit of code actually overlaps in terms of the handled operators, but with this change that could easily be tweaked in the future.

I also want to have this separated out to make experimenting with a eager value info implementation and possibly a ValueTracking-like fixed depth recursion peephole version. There's no reason all four of these can't share a common implementation which reduces the chances of bugs.

Differential Revision: https://reviews.llvm.org/D27294

llvm-svn: 288413