[X86][SSE] Transform truncations between vectors of integers into X86ISD::PACKUS...

[X86][SSE] Transform truncations between vectors of integers into X86ISD::PACKUS/PACKSS operations during DAG combine.

This patch transforms truncation between vectors of integers into
X86ISD::PACKUS/PACKSS operations during DAG combine. We don't do it in
lowering phase because after type legalization, the original truncation
will be turned into a BUILD_VECTOR with each element that is extracted
from a vector and then truncated, and from them it is difficult to do
this optimization. This greatly improves the performance of truncations
on some specific types.

Cost table is updated accordingly.

Differential revision: http://reviews.llvm.org/D14588

git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@256194 91177308-0d34-0410-b5e6-96231b3b80d8