[RISCV] Remove sext_inreg+add/sub/mul/shl isel patterns.

Let the sext_inreg be selected to sext.w. Remove unneeded sext.w
during PostProcessISelDAG.

This gives opportunities for some other isel patterns to match
like the ADDIPair or matching mul with immediate to shXadd.

This becomes possible after D107658 started selecting W instructions
based on users. The sext.w will be considered a W user so isel
will often select a W instruction for the sext.w input and we can
just remove the sext.w. Otherwise we can combine the sext.w with
a ADD/SUB/MUL/SLLI to create a new W instruction in parallel
to the the original instruction.

Reviewed By: luismarques

Differential Revision: https://reviews.llvm.org/D107708