Consts and undefs can be used as different types (common with "0" constant)
so don't copy types from consts/undefs, only to them. It doesn't entirely
solve the problem that the type given to the const could be wrong , but
now the only realistic case is with "0" which is the same when casted to
float, so it doesn't matter for lower_int_to_float.

The other change is to get type information for load input/uniform and
store output, and use that to get correct results.

Signed-off-by: Jonathan Marek <jonathan@marek.ca>
Reviewed-by: Jason Ekstrand <jason@jlekstrand.net>