Aarch64 NCG: Use encoded immediates for literals.

Try to generate

    instr x2, <imm>

instead of

    mov x1, lit
    instr x2, x1

When possible. This get's rid if quite a few redundant
mov instructions.

I believe this causes a metric decrease for LargeRecords as
we reduce register pressure.

-------------------------
Metric Decrease:
    LargeRecord
-------------------------