This patch also changes:
the return type of getGatherCost and the signature of the debug function dumpTreeCosts
to use InstructionCost.
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
See this patch for the introduction of the type:
https://reviews.llvm.org/D91174
Depends on D93049