[MLIR] Split off MLIRExecutionEngineUtils to fix libMLIR.so build (PR54242)

Building libMLIR.so currently fails with:

> /usr/bin/ld: /tmp/ccNzulEA.ltrans39.ltrans.o: in function `(anonymous namespace)::SerializeToHsacoPass::optimizeLlvm(llvm::Module&, llvm::TargetMachine&)':
> /builddir/build/BUILD/llvm-project-15.0.0.src/mlir/lib/Dialect/GPU/Transforms/SerializeToHsaco.cpp:328: undefined reference to `mlir::makeOptimizingTransformer(unsigned int, unsigned int, llvm::TargetMachine*)'

This is because MLIRGPUTransforms depends on MLIRExecutionEngine in
https://github.com/llvm/llvm-project/blob/61bb2e4ea82fc5499a271d70d4537383d1942208/mlir/lib/Dialect/GPU/Transforms/SerializeToHsaco.cpp#L328,
but MLIRExecutionEngine is marked as excluded from libMLIR.so.

However, this code doesn't require the full execution engine: It
only performs middle-end optimization, and does not need any of
the JIT/codegen infrastructure. As such, split off a separate
library MLIRExecutionEngineUtils, which only contains that part
and is not excluded from libMLIR.so.

Fixes https://github.com/llvm/llvm-project/issues/54242.

Differential Revision: https://reviews.llvm.org/D125214