[TableGen] Fix SubRegIndex size computation for concatenated subregs of unknown size

When calculating the size of concatenated subregisters, and at least
one of the subregisters involved has an unknown size (-1), then the
concatenated size should be set to -1 as well.

This bug was found for an out-of-tree target.

Looking at lib/Target the only in-tree target that has a subregister
with unknown size is X86:
  X86RegisterInfo.td:  def sub_mask_0   : SubRegIndex<-1>;

But it looks like sub_mask_0 don't result in any concatenated subreg
index with faulty size if looking at X86SubRegIdxRanges[].

Differential Revision: https://reviews.llvm.org/D138341