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[CGP] Limit distance between overflow math and cmp

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[CGP] Limit distance between overflow math and cmp

Inserting an overflowing arithmetic intrinsic can increase register
pressure by producing two values at a point where only one is needed,
while the second use maybe several blocks away. This increase in
pressure is likely to be more detrimental on performance than
rematerialising one of the original instructions.

So, check that the arithmetic and compare instructions are no further
apart than their immediate successor/predecessor.

Differential Revision: https://reviews.llvm.org/D59024

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