PPCBUG-040: decoder.rs sh64() assembled the XS-form shift amount as
(SH[4:0] << 1) | SH[5] instead of (SH[5] << 5) | SH[4:0]. Every
`sradi` with shift N ∈ 1..=62 executed with a completely wrong shift
count (e.g. shift=32 executed as shift=1).
PPCBUG-560: disasm_goldens.rs rldicl() test helper was encoding sh[5:1]
at PPC bits 16-20 and sh[0] at PPC bit 30 — exactly backwards. The wrong
encoder and wrong decoder cancelled out, hiding PPCBUG-040 from tests.
Fix both together so tests validate ISA-correct encodings.
Co-Authored-By: Claude Opus 4.7 (1M context) <noreply@anthropic.com>
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