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

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.

git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@86486 91177308-0d34-0410-b5e6-96231b3b80d8