GlobalISel: Disallow undef generic virtual register uses

With an undef operand, it's possible for getVRegDef to fail and return
null. This is an edge case very little code bothered to
consider. Proper gMIR should use G_IMPLICIT_DEF instead.

I initially tried to apply this restriction to all SSA MIR, so then
getVRegDef would never fail anywhere. However, ProcessImplicitDefs
does technically run while the function is in SSA. ProcessImplicitDefs
and DetectDeadLanes would need to either move, or a new pseudo-SSA
type of function property would need to be introduced.