[TTI][AArch64][SLP] Sets the cost of an ADD reduction 2xi64 to 2.

2xi64 is the legalized type for wide reductions (like 16xi64) and setting the
cost to 2 makes `load-reduce` and `load-zext-reduce` patterns profitable.

The few performance measurments that I did on an aarch64 machine confirm that
these patterns are actually faster when vectorized.

Differential Revision: https://reviews.llvm.org/D130740