[llvm] [GISel][RISCV] Use isSExtCheaperThanZExt when widening G_ICMP. (PR #120032)

[Matt Arsenault via llvm-commits]

https://github.com/arsenm approved this pull request.

It would be nicer if this was bundled into the requested legalize action 

https://github.com/llvm/llvm-project/pull/120032