[RISCV] Insert a sext_inreg when type legalizing i32 shl by constant on RV64.

Similar to what we do for add/sub/mul.

This can help remove some sext.w. There are some regressions on
some bswap tests, but I have an idea how to fix that for a follow up.

A new PACKW pattern is added to handle the new sext_inreg placement.

Differential Revision: https://reviews.llvm.org/D108663