Fix minor bug in VIEW_CONVERT_EXPR expander

[Eric Botcazou]

> If this is going to be valid now, would you mind updating tree.def?

This isn't "going to be valid now", I didn't change anything apart from fixing 
an obvious buglet in RTH's change that I happen to have run into recently.

The current state of affairs is clearly problematic but isn't new.  Maybe we 
could try and enforce the same-size rule, this should be doable I think.

-- 
Eric Botcazou