Fix LSR compile time hang.

Summary:
Limit number of reassociations in GenerateReassociationsImpl.

Reviewers: qcolombet, mkazantsev

Differential Revision: https://reviews.llvm.org/D46039

From: Evgeny Stupachenko <evstupac@gmail.com>
                         <evgeny.v.stupachenko@intel.com>
llvm-svn: 332426