[CodeGen] Split some functionality from DetectDeadLanes into its own class to be...

[CodeGen] Split some functionality from DetectDeadLanes into its own class to be reused. NFC

This is an alternative to D140382.

This moves the first part of runOnce into a new class that can be
reused by D129735. This encapsulates the ownership of the worklist,
and VRegInfos into the new class. The code for updating the dead
lanes stays in the DetectDeadLanes class.

The new class is created on the stack during runOnMachineFunction
so all the data structures will be deleted after each run.
Previously we only cleared them after each run so the memory might
have stayed allocated across runs. Except for VRegInfo which was
always deleted. Hopefully this allocation change isn't a big deal.

Reviewed By: BeMg

Differential Revision: https://reviews.llvm.org/D141993