Refactor `HasMultiRegRetVal` and `impFixupCallStructReturn`. (#36465)

Refactor `HasMultiRegRetVal` and `impFixupCallStructReturn`. (#36465)

* Fix target definitions.

They were used in asserts only, no changes.

* Fix failures after a recent HW changes.

* Add a const getter for `ReturnTypeDesc` from a call.

Used to make some new methods const as well.

* Refactor `HasMultiRegRetVal` and `impFixupCallStructReturn`.

Delete an unnecessary nested condition and make checks more straightforward.

* Delete an extra `.` in some dumps.

* Add an additional check that `ReturnTypeDesc` is initialized.

* Remove old `const_cast` around `GetReturnTypeDesc`.

* Replace non-const `GetReturnTypeDesc` with other methods.

* Fix uninitialized `gtSpillFlags, gtOtherRegs, gtReturnTypeDesc` in `fgMorphIntoHelperCall`.