PPCBUG-561: Add DecodedInstr::mb_md() to decoder.rs — the correct MD-form
6-bit mask-begin reconstruction (MB[4:0] at PPC bits 21-25, MB[5] at PPC
bit 26). The disassembler already had the correct local formula; this
promotes it to a single source of truth on DecodedInstr.
PPCBUG-046: All 6 doubleword-rotate arms (rldicl, rldicr, rldic, rldimi,
rldcl, rldcr) inlined "(instr.mb() << 1) | ((instr.raw >> 1) & 1)" which
reads SH5 (host bit 1) instead of MB5 (host bit 5). For the canonical
"clrldi r3, r4, 32" zero-extend idiom (mb=32 → MB5=1, MB[4:0]=0), the
wrong formula produced mb=0, making the instruction a no-op and leaving
upper 32 bits of the GPR polluted. Replace all 6 sites with instr.mb_md().
Co-Authored-By: Claude Opus 4.7 (1M context) <noreply@anthropic.com>
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