NFC: Change getUserCost to return InstructionCost

This patch migrates the TTI cost interfaces to return an 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

Depends on D97382

Reviewed By: ctetreau, paulwalker-arm

Differential Revision: https://reviews.llvm.org/D97466