i965: Make split_virtual_grfs() call compact_virtual_grfs().

Post-splitting, VGRFs have a maximum size (MAX_VGRF_SIZE).  This is
required by the register allocator, as we have to create classes for
each size of VGRF.

We can (and do) allocate virtual registers larger than MAX_VGRF_SIZE,
but we must ensure that they are splittable.  split_virtual_grfs()
asserts that the post-splitting register size is in range.

Unfortunately, these trip for completely dead registers which are too
large - we only set split points for live registers.  So dead ones are
never split, and if they happened to be too large, they'd trip asserts.

To fix this, call compact_virtual_grfs() to eliminate dead registers
before splitting.

v2: Add a comment written by Iago.

Cc: mesa-stable@lists.freedesktop.org
Signed-off-by: Kenneth Graunke <kenneth@whitecape.org>
Reviewed-by: Iago Toral Quiroga <itoral@igalia.com>
Reviewed-by: Timothy Arceri <timothy.arceri@collabora.com>

src/mesa/drivers/dri/i965/brw_fs.cpp

@@ -1687,6 +1687,12 @@ fs_visitor::assign_gs_urb_setup()
 void
 fs_visitor::split_virtual_grfs()
 {
+   /* Compact the register file so we eliminate dead vgrfs.  This
+    * only defines split points for live registers, so if we have
+    * too large dead registers they will hit assertions later.
+    */
+   compact_virtual_grfs();
+
    int num_vars = this->alloc.count;
 
    /* Count the total number of registers */