Improve tracking of memory locations in LVN.

Rewrite the tracking of values stored in memory to allow
recognizing the same value after storing it in memory and
loading it back to vreg. Drop reliance on value name
ordering for memory versioning in preparation for GVN.

Also fix a few minor issues in LVN.

Change-Id: Ifabe2d47d669d9ec43942cea6fd157e41af77ec8