PPCBUG-563: Add vx128_4_imm() (PPC bits 11-15) and vx128_4_z() (PPC bits
24-25) accessors to decoder.rs for VX128_4-form instructions.
PPCBUG-315: vrlimi128 was reading z from host bits 16-17 (a subset of IMM)
and mask from host bits 2-5 (a reserved/XO region). Replace with the
correct accessors: z selects which word-lane to start the rotation from
(0-3); IMM is the 5-bit per-lane blend mask.
Co-Authored-By: Claude Opus 4.7 (1M context) <noreply@anthropic.com>
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