nv50/ir: only stick one preret per function

nv50/ir: only stick one preret per function

A function with multiple returns would have had multiple preret settings
at the top of the function. While this is unlikely to have caused issues
since we don't use functions in earnest, it could have in some cases
overflowed the call stack, in case a function had a lot of early
returns.

Signed-off-by: Ilia Mirkin <imirkin@alum.mit.edu>
Reviewed-by: Samuel Pitoiset <samuel.pitoiset@gmail.com>