[TTI] NFC: Change getArithmeticReductionCost 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

This patch is practically NFC, with the exception of an AArch64 SVE related
cost-model change, where we can now return an Invalid cost instead of some
bogus number.

Reviewed By: dmgreen

Differential Revision: https://reviews.llvm.org/D100201