[DAGCombiner][RISCV] Improve computeKnownBits for (smax X, C) where C is non-negative.

If C is non-negative, the result of the smax must also be
non-negative, so all sign bits of the result are 0.

This allows DAGCombiner to remove a zext_inreg in the modified test.
This zext_inreg started as a sext that became zext before type
legalization then was promoted to a zext_inreg.

Reviewed By: reames

Differential Revision: https://reviews.llvm.org/D126896