All ten two-/one-register R6 compact branches, byte-exact vs llvm-mc. The
signed forms share POP26/POP27 (opcodes 22/23) with the pre-R6 BLEZL/BGTZL
and with each other; the decode-entry mask sort tries the more-specific
rt=0 / rs=0 forms first, and a small operand-aware hook in
decode_one_inline recovers BGEZC/BLTZC (rs==rt) from the general BGEC/BLTC.
Where R6 reuses a pre-R6/PSP major opcode (BEQC vs ADDI at opcode 8, etc.)
decode is inherently ISA-mode-dependent and resolves to the legacy form;
the R6 encode side is exact. 281 tests green.
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