util/ra: Don't destroy the graph in ra_allocate()

util/ra: Don't destroy the graph in ra_allocate()

We want to be able to call ra_allocate() and, when it fails, mutate the
graph and try again rather than re-building the graph from scratch.
This commit moves all the scratch bits except the final register
allocation (which is really an out value not scratch) into sub-structs
named "tmp" to make it clear which things are scratch.  It also adds
bits to the ra_select() initialization loop to initialize things (since
we can't trust rzalloc anymore) and copy q_test and forced_reg over.

Reviewed-by: Eric Anholt <eric@anholt.net>