[analyzer] IDC: Add config option; perform the idc check on first “null node”...

[analyzer] IDC: Add config option; perform the idc check on first “null node” rather than last “non-null”.

The second modification does not lead to any visible result, but, theoretically, is what we should
have been looking at to begin with since we are checking if the node was assumed to be null in
an inlined function.

llvm-svn: 176576