LDRB/LDRH/STRB/STRH (post-index, pre-index, register-offset),
LDRSB/LDRSH (register-offset, W and X) and LDRSW (register-offset), plus
the vector pair/unscaled forms LDP_V/STP_V (S/D/Q) and LDUR_V/STUR_V
(S/D/Q). Hand-written, reusing the existing OFFSET_BASE_POST/PRE/REG/S9
addressing encodings; canonical bits taken from llvm-mc (operand fields
zeroed). All 23 representative forms byte-exact vs llvm-mc and
decode-clean; 461 tests green.
(LDARB_X/LDARH_X/STLRB_X/STLRH_X left unimplemented: LDARB/LDARH/STLRB/
STLRH are byte/half acquire-release into a W register with no distinct
64-bit 'X' encoding -- these enum entries are vestigial.)
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