[ConstantRange] Add setSizeSmallerThanOf method.

Summary:
ConstantRange class currently has a method getSetSize, which is mostly used to
compare set sizes of two constant ranges (there is only one spot where it's used
in a slightly different scenario). This patch introduces setSizeSmallerThanOf
method, which does such comparison in a more efficient way. In the original
method we have to extend our types to (BitWidth+1), which can result it using
slow case of APInt, extra memory allocations, etc.

The change is supposed to not change any functionality, but it slightly improves
compile time. Here is compile time improvements that I observed on CTMark:
* tramp3d-v4	-2.02%
* pairlocalalign	-1.82%
* lencod	-1.67%

Reviewers: sanjoy, atrick, pete

Subscribers: llvm-commits

Differential Revision: https://reviews.llvm.org/D31104

llvm-svn: 298236