Use named constant to indicate all lanes, to handle 32 and 64 wide architectures

Summary: Use named constant to indicate all lanes, to handle 32 and 64 wide architectures

Reviewers: ABataev, jdoerfert, grokos, ronlieb

Reviewed By: grokos

Subscribers: ronlieb, openmp-commits

Tags: #openmp

Differential Revision: https://reviews.llvm.org/D68369

llvm-svn: 373793