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

[Amara Emerson via llvm-commits]

aemerson wrote:

Curious: why is it correct to do a sign-extend for an unsigned comparison promotion?

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