[TableGen] Let -register-info-debug dump Offset/Size for each SubRegIndex

This patch adds dumping of the Offset and Size info for each
SubRegIndex printed when using
  llvm-tblgen -gen-register-info -register-info-debug

It also updates the ConcatenatedSubregs.td to check those printouts,
including some new subreg definitions that show short-comings in
how the size is calculated when concatenating subregisters and at
least one has an incomplete size (-1). Today TableGen will just add
sizes together, resulting in MCRegisterInfo::getSubRegIdxSize()
returning a value that isn't -1 even if the combined subregister size
is unknown.

Differential Revision: https://reviews.llvm.org/D138340