The value 35468 changes sign when stored in int16_t:
implicit conversion from 'int' to 'int16_t' (aka 'short')
changes value from 35468 to -30068

This negation requires adding back the original value to compensate.
Shifting the value keeps the value positive and saves a post-vqdmulh
shift.

This technique is used in webp and idct_dequant_full_2x_neon

BUG=b/28027557

Change-Id: I0c5ce09bea170fe08061856c2af6f841a557e0c3