Use continue in the use-processing loop to make it clear what the early exits

Use continue in the use-processing loop to make it clear what the early exits
are, simplify logic, and cause things to not be nested as deeply.  This also
uses MRI->areAliases instead of an explicit loop.

No functionality change, just code cleanup.

git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@23296 91177308-0d34-0410-b5e6-96231b3b80d8