Only generate addcarry node when it is legal.

Summary:
This is a problem uncovered by stage2 testing. ADDCARRY end up being generated on target that do not support it.

The patch that introduced the problem has other patches layed on top of it, so we want to fix the issue rather than revert it to avoid creating a lor of churn.

A regression test will be added shortly, but this is committed as this in order to get the build back to green promptly.

Subscribers: llvm-commits

Differential Revision: https://reviews.llvm.org/D33770

llvm-svn: 304409