[dev.link] cmd/link: experiment new reloc accessors in deadcode pass

[dev.link] cmd/link: experiment new reloc accessors in deadcode pass

There is a small speedup:

(linking cmd/compile)

name       old time/op    new time/op    delta
Deadcode     57.1ms ± 1%    53.5ms ± 1%   -6.44%  (p=0.008 n=5+5)

With this, we don't need a slice to read the relocations, reduce
some allocations.

name       old alloc/op   new alloc/op   delta
Deadcode     4.16MB ± 0%    3.84MB ± 0%   -7.85%  (p=0.008 n=5+5)

Change-Id: Icd41c05682ba3f293a8cb9d2fe818e39d7276e5a
Reviewed-on: https://go-review.googlesource.com/c/go/+/222244
Reviewed-by: Than McIntosh <thanm@google.com>