[GlobalISel] Set, require, and verify Legalized MF property.

RegBankSelect and InstructionSelect run after the legalizer and
require a Legalized function: check that all instructions are legal.

Note that this should be in the MachineVerifier, but it can't use the
MachineLegalizer as it's currently in the separate GlobalISel library.
Note that the RegBankSelect verifier checks have the same layering
problem, but we only use inline methods so end up not needing to link
against the GlobalISel library.

llvm-svn: 277472