Fix correctness check for test_vec_fpextend_double

Summary:
Remove the CHECK-DAG calls introduced in r235341, and add a comment that
this test may break due to scheduling variations.

This patch completes the fix discussed in http://reviews.llvm.org/D8804

Reviewers: dsanders, srhines

Subscribers: llvm-commits

Differential Revision: http://reviews.llvm.org/D9178

llvm-svn: 235530