[AArch64][SVE2] Change the cost of extends with S/URHADD to 0

When SVE2 is enabled, we can combine an add of 1, add & shift right by 1
to a single s/urhadd instruction. If the operands to the adds are extended,
these extends will fold into the s/urhadd and their costs should be 0.

Reviewed By: david-arm, dtemirbulatov

Differential Revision: https://reviews.llvm.org/D157628