[InstCombine] Combine ZExt (B - A) + ZExt(A) to ZExt(B)

Combine ZExt (B - A) + ZExt(A) to ZExt(B)
https://alive2.llvm.org/ce/z/ESUwPi

Reviewed By: spatel

Differential Revision: https://reviews.llvm.org/D139930