[mlir] support reductions in SCF to OpenMP conversion

[mlir] support reductions in SCF to OpenMP conversion

OpenMP reductions need a neutral element, so we match some known reduction
kinds (integer add/mul/or/and/xor, float add/mul, integer and float min/max) to
define the neutral element and the atomic version when possible to express
using atomicrmw (everything except float mul). The SCF-to-OpenMP pass becomes a
module pass because it now needs to introduce new symbols for reduction
declarations in the module.

Reviewed By: chelini

Differential Revision: https://reviews.llvm.org/D107549