[InstCombine] Combine lshr of add -> (a + b < a)

Tries to perform
  (lshr (add (zext X), (zext Y)), K)
  ->  (icmp ult (add X, Y), X)
  where
    - The add's operands are zexts from a K-bits integer to a bigger type.
    - The add is only used by the shr, or by iK (or narrower) truncates.
    - The lshr type has more than 2 bits (other types are boolean math).
    - K > 1

This seems to be a pattern that just comes from OpenCL front-ends, so adding DAG/GISel combines doesn't seem to be worth the complexity.

Original patch D107552 by @abinavpp - adapted to use (a + b < a) instead of uaddo following discussion on the review.
See this issue https://github.com/RadeonOpenCompute/ROCm/issues/488

Reviewed By: spatel

Differential Revision: https://reviews.llvm.org/D138814

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