[CostModel][X86] Fix SLM <2 x i64> icmp costs

SLM is 2 x slower for <2 x i64> comparison ops than other vector types, we should account for this like we do for SLM <2 x i64> add/sub/mul costs.

This should remove some of the SLM codegen diffs in D43582

llvm-svn: 372954