TableGen: Support lanemasks for classes without subregisters

Previously using lanemasks on registers without any subregisters was not
well defined. This commit extends TargetRegisterInfo/tablegen to:
- Report a lanemask of 1 for regclasses without subregisters
- Do the right thing when mapping a 0/1 lanemask from a class without
  subregisters into a class with subregisters in
  TargetRegisterInfo::composeSubRegIndexLaneMasks().

This will be used in the upcoming "DetectDeadLanes" patch.

llvm-svn: 267848