msan: Consider handling add/sub as add

[vitalybuka]

          > consider this an acceptable tradeoff for performance.

if we apply instruction to just allocated memory block completly uninitialzied
sub(Sa, Sb) can produce zero shadow?

Maybe better to apply Add(Sa, Sb) always?

Originally posted by @vitalybuka in #124159 (comment)

[thurstond]

Does this make us an adds company?