[UBSan] Cannot negate smallest negative signed integer

Silence warning Undefined Behavior Sanitzer warning:
runtime error: negation of -9223372036854775808 cannot be represented in type 'int64_t' (aka 'long'); cast to an unsigned type to negate this value to itself

Reviewed By: paulwalker-arm

Differential Revision: https://reviews.llvm.org/D90710