[MCA][InstrBuilder] Correctly mark reserved resources in initializeUsedResources.

This fixes a bug reported by Alex Renda on LLVMDev where mca did not correctly
mark a resource group as "reserved".
(See http://lists.llvm.org/pipermail/llvm-dev/2020-May/141485.html).

The issue was caused by a wrong check in function `initializeUsedResources`.
As a consequence of this, a resource group was left unreserved, and its field
`NumUnits` incorrectly reported an unrealistic number of consumed resource
units.

This patch fixes the issue with the handling of reserved resources in the
InstrBuilder class, and adds a simple test for it.  Ideally, as suggested by
Andy Trick, most of these problems will disappear if in the future we will
introduce a (optional) DelayCycles vector for SchedWriteRes.