Added PositiveInfOrHighestValue and NegativeInfOrLowestValue

[johnplatts]

Added hwy::PositiveInfOrHighestValue<T>(), which returns positive infinity if T is a floating point type or hwy::HighestValue<T>() if T is an integer type.

Also added hwy::NegativeInfOrLowestValue<T>() which returns negative infinity if T is a floating point type or hwy::HighestValue<T>() if T is an integer type.

Also updated EMU128 ReduceMin/ReduceMax to use hwy::PositiveInfOrHighestValue<T>() and hwy::NegativeInfOrLowestValue<T>().

[jan-wassenberg]

Nice improvement, thanks :)