[MLIR][OpenMP] Add support for basic SIMD construct

Patch adds a new operation for the SIMD construct.  The op is designed to be very similar to the existing `wsloop` operation, so that the `CanonicalLoopInfo` of `OpenMPIRBuilder` can be used.

Reviewed By: shraiysh

Differential Revision: https://reviews.llvm.org/D118065