Fabric: `insertedPairs` in `calculateShadowViewMutations` now stores pointers (not values)
Fabric: `insertedPairs` in `calculateShadowViewMutations` now stores pointers (not values)

Summary:
This is a small micro-optimization in Diffing algorithm.
Seems we don't need to store full ShadowView objects in `insertedPairs` map, we can store only pointers to them. That can save memory and CPU cycles because we will not need to store full objects and copy shared pointers (which is somewhat expensive).

Reviewed By: mdvacca

Differential Revision: D15200498

fbshipit-source-id: 2a268c3ee80755555bff3317e10e679be1cf9830

javache/react-native,javache/react-native,janicduplessis/react-native,hammerandchisel/react-native,arthuralee/react-native,javache/react-native,arthuralee/react-native,facebook/react-native,javache/react-native,janicduplessis/react-native,myntra/react-native,pandiaraj44/react-native,exponent/react-native,arthuralee/react-native,hammerandchisel/react-native,exponent/react-native,janicduplessis/react-native,hoangpham95/react-native,janicduplessis/react-native,exponentjs/react-native,hammerandchisel/react-native,exponentjs/react-native,exponentjs/react-native,javache/react-native,janicduplessis/react-native,myntra/react-native,javache/react-native,exponent/react-native,facebook/react-native,facebook/react-native,exponent/react-native,hammerandchisel/react-native,exponent/react-native,pandiaraj44/react-native,exponentjs/react-native,pandiaraj44/react-native,hoangpham95/react-native,exponent/react-native,hoangpham95/react-native,janicduplessis/react-native,javache/react-native,myntra/react-native,myntra/react-native,facebook/react-native,hoangpham95/react-native,exponentjs/react-native,exponentjs/react-native,arthuralee/react-native,javache/react-native,exponent/react-native,facebook/react-native,hammerandchisel/react-native,hammerandchisel/react-native,myntra/react-native,arthuralee/react-native,javache/react-native,janicduplessis/react-native,hoangpham95/react-native,pandiaraj44/react-native,myntra/react-native,exponent/react-native,hammerandchisel/react-native,hoangpham95/react-native,hoangpham95/react-native,facebook/react-native,pandiaraj44/react-native,pandiaraj44/react-native,exponentjs/react-native,myntra/react-native,hammerandchisel/react-native,exponentjs/react-native,pandiaraj44/react-native,hoangpham95/react-native,myntra/react-native,myntra/react-native,facebook/react-native,pandiaraj44/react-native,janicduplessis/react-native,facebook/react-native,facebook/react-native