PPCBUG-565: Add vx128_5_sh() to decoder.rs — 4-bit shift at PPC bits
22-25 (host bits 6-9). The correct MSB is at PPC bit 22 (host bit 9).
PPCBUG-361: vsldoi128 was reading the SH MSB from host bit 4 (PPC bit
27, reserved) instead of host bit 9 (PPC bit 22). All shift amounts >= 8
decoded incorrectly (e.g. shift=8 executed as shift=0). Replace the
inline bit-shuffle with instr.vx128_5_sh().
Also fix vx128_p_perm_assembles_correctly test: replace nonexistent
DecodedInstr::from_raw() calls with struct literal construction.
Co-Authored-By: Claude Opus 4.7 (1M context) <noreply@anthropic.com>
64e8ecbfd0