[AArch64][GlobalISel] Make G_SEXT_INREG legal and add selection support.

We were defaulting to the lower action for this, resulting in SHL+ASHR
sequences. On AArch64 we can do this in one instruction for an arbitrary
extension using SBFM as we do for G_SEXT.

Differential Revision: https://reviews.llvm.org/D81992