cmd/internal/obj: hoist constant slice out of function called in loop

cmd/internal/obj: hoist constant slice out of function called in loop

It's all local to a single file and responsible for 1.7% of total
space allocated summed over compilation of the bent benchmarks.

Showing nodes accounting for 27.16GB, 27.04% of 100.44GB total
Dropped 1622 nodes (cum <= 0.50GB)
Showing top 10 nodes out of 321
      flat  flat%   sum%        cum   cum%
    4.87GB  4.85%  4.85%     4.87GB  4.85%  cmd/compile/internal/objw.init
    4.79GB  4.77%  9.62%     4.81GB  4.79%  runtime.allocm
    4.72GB  4.70% 14.32%     4.72GB  4.70%  cmd/compile/internal/types.newType
    3.10GB  3.09% 17.41%     3.17GB  3.15%  cmd/compile/internal/ssagen.InitConfig
    1.86GB  1.85% 19.26%     2.61GB  2.60%  cmd/compile/internal/ssa.cse

    1.72GB  1.71% 20.97%     2.25GB  2.24%  cmd/internal/obj.(*Link).traverseFuncAux

    1.66GB  1.65% 22.62%     1.66GB  1.65%  runtime.malg
    1.61GB  1.61% 24.23%     1.64GB  1.63%  cmd/compile/internal/ssa.schedule
    1.42GB  1.41% 25.64%     1.42GB  1.41%  cmd/compile/internal/ir.NewNameAt

Change-Id: Id18ee3b83cb23a7042d59714a0c1ca074e7bc7a8
Reviewed-on: https://go-review.googlesource.com/c/go/+/437297
Reviewed-by: Matthew Dempsky <mdempsky@google.com>
Reviewed-by: Cherry Mui <cherryyz@google.com>
TryBot-Result: Gopher Robot <gobot@golang.org>
Run-TryBot: David Chase <drchase@google.com>