[OpenMP] Add flag for preventing the extension to 64 bits for the collapse loop counter

Summary: Introduce a compiler flag for cases when the user knows that the collapsed loop counter can be safely represented using at most 32 bits. This will prevent the emission of expensive mathematical operations (such as the div operation) on the iteration variable using 64 bits where 32 bit operations are sufficient.

Reviewers: ABataev, caomhin

Reviewed By: ABataev

Subscribers: hfinkel, kkwli0, guansong, cfe-commits

Differential Revision: https://reviews.llvm.org/D55928

llvm-svn: 350758