[TableGen][SubtargetEmitter] Add the StartAtCycles field in the WriteRes class.

Conditions that need to be met:

1. count(StartAtCycle) == count(ReservedCycles);
2. For each i: StartAtCycles[i] < ReservedCycles[i];
3. For each i: StartAtCycles[i] >= 0;
4. If left unspecified, the elements are set to 0.

Differential Revision: https://reviews.llvm.org/D150310