[TTI] NFC: Change getFPOpCost 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: c-rhodes

Differential Revision: https://reviews.llvm.org/D100316

GitOrigin-RevId: d84bd951a8d39436b0ca0cd3fa93cacef2f969f1
4 files changed