[ConstantRange] Add fromKnownBits() method

Following the suggestion in D59450, I'm moving the code for constructing
a ConstantRange from KnownBits out of ValueTracking, which also allows us
to test this code independently.

I'm adding this method to ConstantRange rather than KnownBits (which
would have been a bit nicer API wise) to avoid creating a dependency
from Support to IR, where ConstantRange lives.

Differential Revision: https://reviews.llvm.org/D59475

llvm-svn: 356339