ISel/AArch64: custom lower vector ISD::[L]LRINT (#89035)

Since 98c90a13 (ISel: introduce vector ISD::LRINT, ISD::LLRINT; custom
RISCV lowering), ISD::LRINT and ISD::LLRINT now have vector variants,
that are custom lowered on RISCV, and scalarized on all other targets.
Since 2302e4c3 (Reland "VectorUtils: mark xrint as trivially
vectorizable"), lrint and llrint are trivially vectorizable, so all the
vectorizers in-tree will produce vector variants when possible. Add a
custom lowering for AArch64 to custom-lower the vector variants natively
using a combination of frintx, fcvte, and fcvtzs.