fix long long standing bug in enumFromTo_double

enumFromTo_double is part of the fusion specialization machinery of enumFromTo when specialized to Float and Double.

Prior to 7.8, the associated RULE wouldn't fire on the test suite even at O2. With GHC 7.8, the enumFromTo_double
triggers a failure in the Vector test suite.

Special Thanks to Amos Robinson for helping pin down the source of the bug and writing a correct fix.