[LoongArch] Combine ((x & shifted_mask) >> shamt) to bstrpick.{w/d}

This is an improvement to LoongArch codegen. In D127206 we combined
((x >> shamt) & shifted_mask) to bstrpick and here we do a similar
combination when certain conditions are met.

Thanks to @xen0n for reminding me.

Differential Revision: https://reviews.llvm.org/D128191