[RISCV] DAG combine (sra (shl X, 32), 32 - C) -> (shl (sext_inreg X, i32), C).

The sext_inreg can often be folded into an earlier instruction by
using a W instruction. The sext_inreg also works better with our ABI.

This is one of the steps to improving the generated code for this https://godbolt.org/z/hssn6sPco

Reviewed By: asb

Differential Revision: https://reviews.llvm.org/D128843