[TargetLowering][RISCV] Introduce isSExtCheaperThanZExt hook and implement for RISC-V

DAGTypeLegalizer::PromoteSetCCOperands currently prefers to zero-extend 
operands when it is able to do so. For some targets this is more expensive 
than a sign-extension, which is also a valid choice. Introduce the 
isSExtCheaperThanZExt hook and use it in the new SExtOrZExtPromotedInteger 
helper. On RISC-V, we prefer sign-extension for FromTy == MVT::i32 and ToTy == 
MVT::i64, as it can be performed using a single instruction.

Differential Revision: https://reviews.llvm.org/D52978

llvm-svn: 347977