Revised RangeConstraintManager based on several discussions with Ben Laurie and

Revised RangeConstraintManager based on several discussions with Ben Laurie and
Zhongxing Xu. The resultant code is less than 1/2 the size of the original.

Key highlights:

- All CouldBeXXX methods have been removed. Checking for feasibility is now just
  done in the AddXXX methods.

- RangeSets now represent "all possible values" explicitly as the range set {
  [min, max] } instead of the empty set. The empty set now represents "no
  feasible values". This change consolidated much of the core algorithm to only
  have one code path instead of alternate paths that considered the empty set to
  represent "all possible falues."

git-svn-id: https://llvm.org/svn/llvm-project/cfe/trunk@64787 91177308-0d34-0410-b5e6-96231b3b80d8