change to use correct type
change to use correct type

This commit changes to use uint32_t for cost (always non-negative),
and promote to int64_t before calculation of the savings.

This fixes an integer overflow.

Change-Id: I71c2580d188cc79d2d8069241d0353cf331b5c83

GrokImageCompression/aom,GrokImageCompression/aom,GrokImageCompression/aom,luctrudeau/aom,shacklettbp/aom,smarter/aom,GrokImageCompression/aom,shacklettbp/aom,shacklettbp/aom,mbebenita/aom,GrokImageCompression/aom,shacklettbp/aom,GrokImageCompression/aom,mbebenita/aom,luctrudeau/aom,luctrudeau/aom,mbebenita/aom,luctrudeau/aom,mbebenita/aom,mbebenita/aom,smarter/aom,shacklettbp/aom,mbebenita/aom,luctrudeau/aom,luctrudeau/aom,smarter/aom,smarter/aom,smarter/aom,smarter/aom,mbebenita/aom,mbebenita/aom,shacklettbp/aom,mbebenita/aom