[SLP][AArch64] Implement lookahead operand reordering score of splat loads for AArch64

The original patch (https://reviews.llvm.org/D121354) targets x86 and adjusts
the lookahead score of splat loads ad they can be done by the `movddup`
instruction that combines the load and the broadcast and is cheap to execute.

A similar issue shows up on AArch64. The `ld1r` instruction performs a broadcast
load and is cheap to execute.

This patch implements the TargetTransformInfo hooks for AArch64.

Differential Revision: https://reviews.llvm.org/D123638