[ARM] Add MVE add.sat costs

This adds some basic MVE sadd_sat/ssub_sat/uadd_sat/usub_sat costs,
based on when the instruction is legal. With smaller than legal types
that are promoted we generate shr(qadd(shl, shl)), so the cost is 4
appropriately.

Differential Revision: https://reviews.llvm.org/D94958