[GuardWidening] Introduce range check merging

Sequences of range checks expressed using guards, like

  guard((I - 2) u< L)
  guard((I - 1) u< L)
  guard((I + 0) u< L)
  guard((I + 1) u< L)
  guard((I + 2) u< L)

can sometimes be combined into a smaller sequence:

  guard((I - 2) u< L AND (I + 2) u< L)

if we can prove that (I - 2) u< L AND (I + 2) u< L implies all of checks
expressed in the previous sequence.

This change teaches GuardWidening to do this kind of merging when
feasible.

llvm-svn: 270151