[LiveDebugValues] Handle spill locations with a fixed and scalable component.

This patch fixes the two LiveDebugValues implementations
(InstrRef/VarLoc)Based to handle cases where the StackOffset contains
both a fixed and scalable component.

This depends on the `TargetRegisterInfo::prependOffsetExpression` being
added in D90020. Feel free to leave comments on that patch if you have them.

Reviewed By: djtodoro, jmorse

Differential Revision: https://reviews.llvm.org/D90046