[X86] Add some test cases for 512-bit truncate to 128-bits with...

[X86] Add some test cases for 512-bit truncate to 128-bits with min-legal-vector-width=0 and prefer-vector-width=256.

We currently split the 512 type, truncate each half to 128 bits,
concatenate them, and then truncate again. Probably better to
truncate each half to 64-bits and then concat the results
using vpunpcklqdq.

llvm-svn: 367600