AVX/AVX2: Move the code that lowers vector-trunc from a DAGCo-hook to custom lowering hook.

The vector truncs were scalarized during LegalizeVectorOps, later vectorized again by some DAGCombine optimization
and finally, lowered by a dagcombing optimization. Now, they are properly lowered during LegalizeVectorOps.
No new testcase because the original testcases still work.

llvm-svn: 171146