Reduce memory usage in GVN

Implement recycling of ValueSet data structures which the GVN
algorithm will not access any more.

Savings depend on the shape of the graph, but can be as high as 93%.
Peak memory usage for GSA drops from 32MB to 26MB, compile times seem
unaffected.

Bug: 28173563
Bug: 28287086

Change-Id: If227177449bc90ad24fa68c37b0c2615924af1ed