cmd/compile: preload multi-instruction constants before loops on arm64

The register allocator never preloads rematerializeable values into
registers before a loop, so a 64-bit constant that needs MOVD plus up
to three MOVKs on arm64 is rebuilt on every iteration. Preload such
constants when a register is free, as is already done for other
values live into the loop.

Single-instruction constants are unchanged.

Neoverse N1, repro from the issue (HashVar loads the constant from a
variable and is unaffected):

        │    base     │                 new                 │
        │   sec/op    │   sec/op     vs base                │
Hash      5.165µ ± 0%   4.128µ ± 0%  -20.09% (p=0.000 n=10)
HashVar   4.128µ ± 0%   4.129µ ± 0%        ~ (p=0.778 n=10)

For #81755

Change-Id: I87c5779b24057cd59833c2afc32c7ddef06a29df
Reviewed-on: https://go-review.googlesource.com/c/go/+/840405
Reviewed-by: Keith Randall <khr@google.com>
LUCI-TryBot-Result: golang-scoped@luci-project-accounts.iam.gserviceaccount.com <golang-scoped@luci-project-accounts.iam.gserviceaccount.com>
Auto-Submit: Keith Randall <khr@golang.org>
Reviewed-by: David Chase <drchase@google.com>
Reviewed-by: Keith Randall <khr@golang.org>