svga: move temp register index assertions

svga: move temp register index assertions

The assertion recently added in dst_register() was invalid because that
function is also (suprisingly) used to declare constant registers.

Move the assertion to the callers where we're really creating temp
registers and add some code to prevent emitting invalid temp register
indexes for release builds.

Also, update the comment for get_temp().  It didn't return -1 if it
ran out of registers and none of the callers checked for that.

Reviewed-by: Jose Fonseca <jfonseca@vmware.com>