[SystemZ] Fix addcarry of addcarry of const carry (PR42606)

This fixes https://bugs.llvm.org/show_bug.cgi?id=42606 by extending
D64213. Instead of only checking if the carry comes from a matching
operation, we now check the full chain of carries. Otherwise we might
custom lower the outermost addcarry, but then generically legalize
an inner addcarry.

Differential Revision: https://reviews.llvm.org/D64658

llvm-svn: 365949