NFC: Migrate SpeculateAroundPHIs to work on InstructionCost
This patch migrates cost values and arithmetic to work on InstructionCost.
When the interfaces to TargetTransformInfo are changed, any InstructionCost
state will propagate naturally.
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: ctetreau
Differential Revision: https://reviews.llvm.org/D95353