[CostModel][X86] Improve AVX1/AVX2 truncation costs

Based off the worse case numbers generated by D103695, we were overestimating the cost of a number of vector truncations:

AVX2: v2i32->v2i8, v2i64->v2i16 + v4i64->v4i32
AVX1: v2i32->v2i8, v4i64->v4i16 + v16i16->v16i8

Once we have a working set of conversion costs, the intention is to cleanup the tables and use legalized types a lot more to reduce the number of entries we currently have.