[LoongArch] Add a hook to sign extend i32 ConstantInt operands of phis on LA64 (#93813)

Materializing constants on LoongArch is simpler if the constant is sign
extended from i32. By default i32 constant operands of phis are zero
extended.
    
This patch adds a hook to allow LoongArch to override this for i32. We
have an existing isSExtCheaperThanZExt, but it operates on EVT which we
don't have at these places in the code.