[ConstantRange] Remove costly udivrem from ConstantRange::truncate

Truncate currently uses a udivrem call which is going to be slow particularly for larger than 64-bit widths.

As far as I can tell all we were trying to do was modulo LowerDiv by (MaxValue+1) and make sure whatever value was effectively subtracted from LowerDiv was also subtracted from UpperDiv.

This patch recognizes that MaxValue+1 is a power of 2 so we can just use a bitwise AND to accomplish a modulo operation or isolate the upper bits.

Differential Revision: https://reviews.llvm.org/D32672

llvm-svn: 304733