Subzero: Count lookups of each constant value in the constant pool.

With "-verbose cpool", at the end it dumps counts of the number of times each pooled constant was looked up.  The output is grouped by type and sorted by constant value, but it's easy to run it through "sort -nr" to see it ordered by popularity.

BUG= none
R=jpp@chromium.org

Review URL: https://codereview.chromium.org/1768173002 .