[X86] Fix several issues related to X86's psadbw instruction.

This patch fixes the following issues:

1. Fix the return type of X86psadbw: it should not be the same type of inputs.
   For vNi8 inputs the output should be vMi64, where M = N/8.
2. Fix the return type of int_x86_avx512_psad_bw_512 accordingly.
3. Fix the definiton of PSADBW, VPSADBW, and VPSADBWY accordingly.
4. Adjust the return type when building a DAG node of X86ISD::PSADBW type.
5. Update related tests.


Differential revision: http://reviews.llvm.org/D14897

llvm-svn: 254010