GCC presently enables the loop vectorizer at lower optimization levels 
for OpenMP loops with the "simd" specifier than it does for loops 
without it.  The "simd" specifier isn't defined to be purely an 
optimization hint to the compiler; it also has semantic effects like 
changing the privatization of the loop variable.  It seems reasonable to 
decouple the additional vectorization from those semantic effects and 
apply it also to work-sharing loops without the "simd" specifier at the 
same optimization levels.

I've tested this patch on x86_64-linux-gnu-amdgcn, plain 
x86_64-linux-gnu, and aarch64-linux-gnu.  OK for mainline?

-Sandra