[ConstantRange] Shl considers full-set shifting to last bit position.

if we do SHL of two 16-bit ranges like [0, 30000) with [1,2) we get
"full-set" instead of what I would have expected [0, 60000) which is
still in the 16-bit unsigned range.

This patch changes the SHL algorithm to allow getting a usable range
even in this case.

Differential Revision: https://reviews.llvm.org/D57983

llvm-svn: 357854