Move VM_Operation to vmOperation.hpp

vmOperations.hpp declares the VM_Operation class, as well as a hodge podge of subclasses such as VM_ForceSafepoint, VM_DeoptimizeFrame.

Out of the 1000 hotspot .o files, about 680 include vmOperations.hpp (mostly transitively). In most cases, they just need to use the VM_Operation class.

So we should move VM_Operation to its own header: vmOperation.hpp (no "s").

After the refactoring, vmOperations.hpp is included only 64 times. The inclusion count of threadSMR.hpp is also reduced from 687 to 99. HotSpot build time is improved by about 0.4%.