Specialise constant-mask swizzle for lsx and lasx - #1422
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On lsx the only new case is the identity mask, which returns the batch unchanged. Every other mask takes the runtime path as before. On lasx there are four cases. An identity mask again returns the batch unchanged. The other three exist because xvshuf.b only shuffles within each 128 bit lane, so permuting bytes across the whole register is the expensive case and worth avoiding. When no element crosses a lane, a single xvshuf.b does the whole job, once each index is reduced to an offset within its own lane. When every element reads from a single lane, that lane is first broadcast to both lanes with xvpermi.q, after which the same single xvshuf.b applies. Any other mask takes the runtime path as before. Those three cases apply only where the element type is one byte. Wider element types get the identity check and nothing else.
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On lsx the only new case is the identity mask, which returns the batch unchanged. Every other mask takes the runtime path as before.
On lasx there are four. An identity mask again returns the batch unchanged. The other three exist because xvshuf.b only shuffles within each 128 bit half of the register, so permuting bytes across the whole register is the expensive case and worth avoiding. When no element reads from the other half, a single xvshuf.b does the whole job, with each index taken modulo the half width so that it addresses its own half. When every element reads from the low half, that half is first broadcast to both halves with xvpermi.q 0x00, after which the same single xvshuf.b applies; a mask reading only the high half is the same with 0x11. Any other mask takes the runtime path as before.
Those three cases apply only where the element type is one byte. Wider element types get the identity check and nothing else.