BZ/BNZ .B/.H/.W/.D/.V (branch if any/all elements zero/non-zero): a
specgen branch emitter that derives the opcode+Wt bits then marks the
16-bit PC-relative offset variable, reusing the existing REL16/BRANCH_16
relocation machinery. The offset is emitted as a relocation (label
target). 10 forms, opcode+Wt byte-exact vs llvm-mc and decode-clean.
The R6 two-/one-register compact branches (BEQC/BNEC/BLTC/BGEC/.../BLTZC)
are deferred: they share POP major opcodes disambiguated only by the
rs/rt relationship, which the opcode+mask decode model can't express
without operand-aware logic. 281 tests green.
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