[gomp4] ptx reduction simplification

[Nathan Sidwell]

I've committed this patch to rework and simplify the ptx backend's 
implementation of the reduction lowering hooks.

The current implementation had a number of bugs (multiple 
gimplify_push_contexts, for instance), and overcomplicated in a number of ways.

Most of the changes are straightforwards.  The vector initializer didn't need to 
create 2 new BBs -- just one for the intialization path.

nathan
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