[llvm] [RISCV] Add isel special case for (and (shl X, c2), c1) -> (slli_uw (srli x, c4-c2), c4). (PR #91638)

[Yingwei Zheng via llvm-commits]

dtcxzyw wrote:

Emm, https://github.com/llvm/llvm-project/pull/91626 seems to perform better than this patch. BTW this patch converts some `slli` insts into `slli.uw`, which makes them less compressible.


https://github.com/llvm/llvm-project/pull/91638