i965/fs: Dump IR when fatally not compiling due to bad register spilling.

It should never happen, but it does, and at this point, you're going to
_mesa_problem() and abort() (unless it's just in precompile).  Give the
developer something to look at.

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

@@ -458,7 +458,8 @@ fs_visitor::assign_regs()
       int reg = choose_spill_reg(g);
 
       if (reg == -1) {
-	 fail("no register to spill\n");
+         fail("no register to spill:\n");
+         dump_instructions();
       } else if (dispatch_width == 16) {
 	 fail("Failure to register allocate.  Reduce number of live scalar "
               "values to avoid this.");