The prefix fields were decoded by hand in the interpreter, the IR frontend,
JitState, MIPSState and the disassembler, with three copies of the constant
table. They now share small decoders and one table, in MIPS.h. The backends'
Vec4Shuffle lane decoding gets a helper of its own in IRInst.h.
Co-Authored-By: Claude Opus 5.5 <noreply@anthropic.com>
Vec4ClampToZero and Vec2ClampToZero only ever fed Vec4Pack31To8 and
Vec2Pack31To16, for vi2uc and vi2us. The packs now clamp negative lanes
to zero themselves, which saves an op and a vector temp, and lets x64
clamp with PACKUSWB's saturation after an arithmetic shift.
While at it, RISC-V compiles Vec2Unpack16To31, Vec2Pack31To16 and
Vec4Pack32To8, and LoongArch Vec2Unpack16To31, Vec2Pack31To16 and the
non-LSX Vec4Pack32To8, all of which went to the IR interpreter.
LoongArch's Vec2Pack32To16 and Vec2Unpack16To32 now take their scalar
path with LSX too, instead of falling back.
Co-Authored-By: Claude Opus 5.5 (1M context) <noreply@anthropic.com>
