[CostModel]Migrate getTreeCost() to use InstructionCost

This patch changes the type of cost variables (for instance: Cost, ExtractCost,
SpillCost) to use InstructionCost.
This patch also changes the type of cost variables to InstructionCost in other
functions that use the result of getTreeCost()
This patch is part of a series of patches to use InstructionCost instead of
unsigned/int for the cost model functions.

See this thread for context:
http://lists.llvm.org/pipermail/llvm-dev/2020-November/146408.html

Depends on D91174

Differential Revision: https://reviews.llvm.org/D93049