TableGen: Support lanemasks for classes without subregisters

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.

git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@267848 91177308-0d34-0410-b5e6-96231b3b80d8