[SelectionDAG][RISCV] Make RegsForValue::getCopyToRegs explicitly zero_extend constants.

ComputePHILiveOutRegInfo assumes that constant incoming values to
Phis will be zero extended if they aren't a legal type. To guarantee
that we should zero_extend rather than any_extend constants.

This fixes a bug for RISCV where any_extend of constants can be
treated as a sign_extend.

Differential Revision: https://reviews.llvm.org/D122053