[MLIR][Mem2Reg] Fix multi slot handling & move retry handling (#91464)

This commit fixes Mem2Regs mutli-slot allocator handling and extends the
test dialect to test this.

Additionally, this modifies Mem2Reg's API to always attempt a full
promotion on all the passed in "allocators". This ensures that the pass
does not require unnecessary walks over the regions and improves caching
benefits.