[AArch64] Only mark cost 1 perfect shuffles as legal

The perfect shuffle tables encode a cost of either 0 (a nop-copy) or 1
(a single instruction) with a cost encoding of 0 in the upper 2 bits.
All perfect shuffles with any cost are then marked as legal shuffles
though (the maximum encoded cost is 3), which can confuse the DAG
combiner into thinking the shuffles are cheaper than the should be.

Limiting legal shuffles to single instructions seems to do better in
most case, producing less instructions for complex shuffles. There are
some cases that now become tbl, which may be better or worse depending
on whether the instruction is in a loop and the tbl load can be hoisted
out.

Differential Revision: https://reviews.llvm.org/D123377