[GlobalISel][AArch64] Combine G_SEXT_INREG + right shift -> G_SBFX

Basically a port of isBitfieldExtractOpFromSExtInReg in AArch64ISelDAGToDAG.

This is only done post-legalization for now. Once the legalizer knows how to
decompose these back into shifts, this requirement can probably be removed.

Differential Revision: https://reviews.llvm.org/D99230