nir: fix dangling ssadef->name ptrs

nir: fix dangling ssadef->name ptrs

In many places, the convention is to pass an existing ssadef name ptr
when construction/initializing a new nir_ssa_def.  But that goes badly
(as noticed by garbage in nir_print output) when the original string
gets freed.

Just use ralloc_strdup() instead, and add ralloc_free() in the two
places that would care (not that the strings wouldn't eventually get
freed anyways).

Also fixup the nir_search code which was directly setting ssadef->name
to use the parent instruction as memctx.

Signed-off-by: Rob Clark <robclark@freedesktop.org>
Reviewed-by: Jason Ekstrand <jason@jlekstrand.net>