This provides the `disable-schedmodel-in-sched-mi` flag. Using this, we
will disable the SchedModel / Itineraries during scheduling. This has
the effect of not using any latency / hardware resource information for
scheduling decisions.

We have the `schedmodel` flag, but this disables the `SchedModel` for
all passes. This allows disabling only for scheduling while preserving
the behavior of other passes (e.g. MachineLICM). This is conceptually
similar to other flags like `enable-aa-sched-mi`

Identified with misc-include-cleaner.

Reverts llvm/llvm-project#90982

NIter was only declared in !NDEBUG, and only used for assertions - so it
was correct that it was incremented inside the assertion. (& in fact now
the non-asserts build fails, because the variable is incremented even
though it isn't declared)

Modified the assert statement

Follow up on a post-commit review of 9468de4 (TargetInstrInfo: make
getOperandLatency return optional (NFC)) by Bjorn Pettersson to fix a
couple of things that are not NFC:

- std::optional<T>::operator<= returns true if the first operand is a
std::nullopt and second operand is T. Fix a couple of places where we
assumed it would return false.
- In TargetSchedule, computeInstrCost could take another codepath,
returning InstrLatency instead of DefaultDefLatency. Fix one instance
not accounting for this behavior.

getOperandLatency has the following behavior: it returns -1 as a special
value, negative numbers other than -1 on some target-specific overrides,
or a valid non-negative latency. This behavior can be surprising, as
some callers do arithmetic on these negative values. Change the
interface of getOperandLatency to return a std::optional<unsigned> to
prevent surprises in callers. While at it, change the interface of
getInstrLatency to return unsigned instead of int.

This change was inspired by a refactoring in
TargetSchedModel::computeOperandLatency.

The option is used to force the use of resource intervals
in the machine scheduler, effectively ignoring the value of
`EnableIntervals` in the instance of the `SchedMachineModel`.

Reviewed By: anemet

Differential Revision: https://reviews.llvm.org/D156540

Change MCInstrDesc::operands to return an ArrayRef so we can easily use
it everywhere instead of the (IMHO ugly) opInfo_begin and opInfo_end.
A future patch will remove opInfo_begin and opInfo_end.

Also use it instead of raw access to the OpInfo pointer. A future patch
will remove this pointer.

Differential Revision: https://reviews.llvm.org/D142213

This is a (fixed) recommit of https://reviews.llvm.org/D121169

after:  1061034926
before: 1063332844

Discourse thread: https://discourse.llvm.org/t/include-what-you-use-include-cleanup
Differential Revision: https://reviews.llvm.org/D121681