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
294fdd9903