The code is applying a bias (shift) to inputs which is wrong.  The
index in case 2 should be i + 2 not i + 4.  We can simplify the code a
little and remove the if statement, reducing 7 lines to 3 in each of
the 3 cases.

The testcase works with the patch, and fails without.  The patch
reduces GCC C testsuite failures from 2174 to 2108 (-66).

Jim