Replace the x64 STVL/STVR lowering with explicit byte stores that preserve the
guest-visible vector byte order.
The previous vector shuffle/blend path could write incorrect bytes for partial
vector stores, which broke optimized guest memcpy implementations using
stvlx/stvrx for unaligned heads and tails. In 4156081C this corrupted skinned
model render command data and eventually caused render-thread crashes.
Add regression coverage for StoreVectorLeft/StoreVectorRight across common
unaligned offsets, plus a memcpy-head style case matching the affected guest
instruction pattern.
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