[AArch64] Eliminate XTN/SSHLL for vector splats (#180913)
Combine:
sext(duplane(insert_subvector(undef, trunc(X), 0), idx))
Into:
duplane(X, idx)
This avoids XTN/SSHLL instruction sequences that occur when splatting
elements from boolean vectors after type legalization, which is common
when using shufflevector with comparison results.
