Optimizing: Add comments to `HInstruction::Add{,Env}UseAt()`.

This is a follow-up for
    https://android-review.googlesource.com/3497214
to add some explanatory comments.

Test: m test-art-host-gtest
Test: testrunner.py --host --optimizing
Bug: 181943478
Change-Id: Ibe759e2851d3d6c6c92a41c33e30798f37687b9f
1 file changed