[TTI] NFC: Change getTypeLegalizationCost 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: sdesmalen, kparzysz

Differential Revision: https://reviews.llvm.org/D101533