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

[Matt Arsenault via llvm-commits]

arsenm wrote:

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

eq / ne / true / false aren't really signed or unsigned, you just need to treat the high bits consistently in both operands 

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