[NFC][InstructionCost] Use InstructionCost in lib/Transforms/IPO/IROutliner.cpp

In places where we call a TTI.getXXCost() function I have changed
the code to use InstructionCost instead of unsigned. This is in
preparation for later on when we will change the TTI interfaces
to return InstructionCost.

See this patch for the introduction of the type: https://reviews.llvm.org/D91174
See this thread for context: http://lists.llvm.org/pipermail/llvm-dev/2020-November/146408.html

Differential Revision: https://reviews.llvm.org/D94427