[DAGCombiner] Don't allow addcarry if the carry producer is illegal.

[DAGCombiner]  Don't allow addcarry if the carry producer is illegal.

getAsCarry() checks that the input argument is a carry-producing node before
allowing a transformation to addcarry. This patch adds a check to make sure
that the carry-producing node is legal. If it is not, it may not remain in a
form that is manageable by the target backend. The test case caused a
compilation failure during instruction selection for this reason on SystemZ.

Patch by Ulrich Weigand.

Review: Sanjay Patel
https://reviews.llvm.org/D59822

git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@357052 91177308-0d34-0410-b5e6-96231b3b80d8