[TTI] NFC: Change getArithmeticInstrCost 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

Reviewed By: dmgreen

Differential Revision: https://reviews.llvm.org/D100317

GitOrigin-RevId: 4f42d873c20291077f5a1ed37b102330d505f00d
23 files changed