[X86] Always assign reassoc flag for intrinsics *reduce_add/mul_ps/pd.

Intrinsics *reduce_add/mul_ps/pd have assumption that the elements in
the vector are reassociable. So we need to always assign the reassoc
flag when we call _mm_reduce_* intrinsics.

Reviewed By: spatel

Differential Revision: https://reviews.llvm.org/D96231