[AArch64] Refactor immediate details out of add/sub tblgen class (NFCI)

Bits [23-22] are used in Add and Sub to specify the shift. The value of the
shift field must be 0x; values of 1x are unallocated. MTE adds some instructions
that use such encodings, and this patch refactors the Add/Sub class so that
another class could derive from this one to implement other encodings and other
formats of bitfields.

Patch by Pablo Barrio!

Differential revision: https://reviews.llvm.org/D52489

llvm-svn: 343231