Fixing the asymmetry in the array field_types_merge_rules[][]
which caused data loss when mixing FLOAT + BIGINT in UNIONs
or hybrid functions:

1. FLOAT  + INT    = DOUBLE
2. FLOAT  + BIGINT = FLOAT
3. INT    + FLOAT  = DOUBLE
4. BIGINT + FLOAT  = DOUBLE

Now FLOAT + BIGINT (as in #2) also produces DOUBLE, like the cases #1,#3,#4 do.