Fix private constructor for ScaledNumber.

Summary:
The private constructor for ScaledNumber was using uint64_t instead of
DigitsT. This was preventing instantiations of ScaledNumber with
anything other than uint64_t types.

In implementing the tests, I ran into another issue. Operators >>= and
<<= did not have variants for accepting other ScaledNumber as the shift
argument. This is expected by the SCALED_NUMBER_BOP.

It makes no sense to allow shifting a ScaledNumber by another
ScaledNumber, so the patch includes two new templates for shifting
ScaledNumbers.

Reviewers: dexonsmith

Subscribers: llvm-commits

Differential Revision: http://reviews.llvm.org/D9350

llvm-svn: 236232