[AArch64] Spot SBFX-compatbile code expressed with sign_extend_inreg.

We were assuming all SBFX-like operations would have the shl/asr form, but often
when the field being extracted is an i8 or i16, we end up with a
SIGN_EXTEND_INREG acting on a shift instead.

This is a port of r213754 from ARM to AArch64.

llvm-svn: 271677