[OPENMP] Support relational-op != (not-equal) as one of the canonical

forms of random access iterator
    
In OpenMP 4.5, only 4 relational operators are supported: <, <=, >, 
and >=.  This work is to enable support for relational operator 
!= (not-equal) as one of the canonical forms.

Patch by Anh Tuyen Tran
    
Differential Revision: https://reviews.llvm.org/D54441

llvm-svn: 347405