[SLPVectorizer]Migrate getEntryCost to return InstructionCost

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

Differential Revision: https://reviews.llvm.org/D93127