[RISCV][SelectionDAG] Add a hook to sign extend i32 ConstantInt operands of phis on RV64.

Materializing constants on RISCV 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 RISCV 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.

Reviewed By: efriedma

Differential Revision: https://reviews.llvm.org/D122951