[PATCH] ValueTracking: Teach isKnownToBeAPowerOfTwo that a power-of-two-or-zero + the same power-of-two-or-zero is a power-of-two-or-zero

[David Majnemer]

Attached is a patch to strengthen isKnownToBeAPowerOfTwo's analysis on add
instructions.

It calls into ComputeMaskedBits to figure out which bits are set on both
add operands and determines if the value is a power-of-two-or-zero or not.

-- 
David Majnemer
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