ExecutionEngine OptUtils: support -On flags in string-based initialization

Original implementation of OutUtils provided two different LLVM IR module
transformers to be used with the MLIR ExecutionEngine: OptimizingTransformer
parameterized by the optimization levels (similar to -O3 flags) and
LLVMPassesTransformer parameterized by the string formatted similarly to
command line options of LLVM's "opt" tool without support for -O* flags.
Introduce such support by declaring the flags inside the parser and by
populating the pass managers similarly to what "opt" does.  Remove the
additional flags from mlir-cpu-runner as they can now be wrapped into
`-llvm-opts` together with other LLVM-related flags.

PiperOrigin-RevId: 236107292