Forward port of Hcal pre-mixing from PR4325

This is forward port by hand of PR 4325, which cannot be merged due to random number thread safity modifications in 7_1_X.

This PR fix pre-mixing for Hcal.

For some reason its peak up PR 4306, which is being validated and is OK but should not go together.
@civanch , which PR changed this in 70X? Is this supposed to work with any data ("old", "new"), could you please explain the intended change 0x3FF -> 0x7FF?