AVX512: Fix LowerMSCATTER() return value.

Bug description:
  The bug was discovered when test was compiled with -O0.
  In case scatter result is DAG root , VectorLegalizer failed (assert) due to LowerMSCATTER() return kmask as result.
Change LowerMSCATTER() to return chain as original node do.

Differential Revision: http://reviews.llvm.org/D17331

llvm-svn: 261090