…(trunci) expansion
This revision adds a rewrite for sequences of vector `bitcast(trunci)`
to use a more efficient sequence of vector operations comprising
`shuffle` and `bitwise` ops.
Such patterns appear naturally when writing quantization /
dequantization functionality with the vector dialect.
The rewrite performs a simple enumeration of each of the bits in the
result vector and determines its provenance in the pre-trunci vector.
The enumeration is used to generate the proper sequence of `shuffle`,
`andi`, `ori` followed by an optional final `trunci`/`extui`.
The rewrite currently only applies to 1-D non-scalable vectors and bails
out if the final vector element type is not a multiple of 8. This is a
failsafe heuristic determined empirically: if the resulting type is not
an even number of bytes, further complexities arise that are not
improved by this pattern: the heavy lifting still needs to be done by
LLVM.
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