[GlobalISel] Add Legalized MachineFunction property.

Legalized: The MachineLegalizer ran; all pre-isel generic instructions
have been legalized, i.e., all instructions are now one of:
  - generic and always legal (e.g., COPY)
  - target-specific
  - legal pre-isel generic instructions.

This lets us enforce certain invariants across passes.
This property is GlobalISel-specific, but is always available.

llvm-svn: 277470