[OPENMP] Loop collapsing and codegen for 'omp simd' directive.

This patch implements collapsing of the loops (in particular, in
presense of clause 'collapse'). It calculates number of iterations N
and expressions nesessary to calculate the nested loops counters
values based on new iteration variable (that goes from 0 to N-1)
in Sema. It also adds Codegen for 'omp simd', which uses
(and tests) this feature.

Differential Revision: http://reviews.llvm.org/D5184