[RISCV] Insert sext_inreg when type legalizing add/sub/mul with constant LHS.

We already do this for non-constants RHS. This just removes the
special case. I believe the special case may have been needed
because the ANY_EXTEND of a constant used to create zero extended
constants, but we recently changed that to produce sign extended
constants.

D107658 is needed to prevent some regressions.

Reviewed By: luismarques

Differential Revision: https://reviews.llvm.org/D107697