[RISCV] Fold (sra (sext_inreg (shl X, C1), i32), C2) -> (sra (shl X, C1+32), C2+32).

The former pattern will select as slliw+sraiw while the latter
will select as slli+srai. This can enable the slli+srai to be
compressed.

Differential Revision: https://reviews.llvm.org/D129688