Fix an issue where the ordering of blocks within a function could lead to different constraint

graphs being produced.  The cause was that we were incorrectly marking sigma instructions as
processed after handling the sigma-specific constraints for them, potentially neglecting to
process them as normal instructions as well.

Unfortunately, the testcase that inspired this still doesn't work because of a bug in the solver,
which is next on the list to debug.

llvm-svn: 86486