Yet another DOM memory improvement

[Jeffrey A Law <law at redhat dot com>]

In an earlier message I indicated that moving the vrp_data information
into the SSA_NAME itself probably wasn't that good of an idea.  I also
suggested that a hash table might be a better means to record that
data since typically it's going to be rather sparse.

This change turns that varray into a hash table per those ideas.  The
memory savings is substantial on the order of many megabytes for
Gerald's testcase -- even though we have to allocate a two word
structure for each hash table element (I did my initial measurements
with those structures in GGC memory, but for the final version I moved
them into malloc'd memory).

Bootstrapped and regression tested on i686-pc-linux-gnu.

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