[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
git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@343231
91177308-0d34-0410-b5e6-
96231b3b80d8
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