[MISched] Do not erase resource booking history for subunits.

When dealing with the subunits of a resource group, we should reset
the subunits availability at the first avaiable cycle of the resource
that contains the subunits. Previously, the reset operation was
returning cycle 0, effectively erasing the booking history of the
subunits.

Without this change, when using intervals for models have make use of
subunits, the erasing of resource booking for subunits can raise the
assertion "A resource is being overwritten" in
`ResourceSegments::add`. The test added in the patch is one of such
cases.

Reviewed By: andreadb

Differential Revision: https://reviews.llvm.org/D156530