runtime: specialize heapSetType

runtime: specialize heapSetType

Last CL we separated mallocgc into several specialized paths. Let's
split up heapSetType too. This will make the specialized heapSetType
functions inlineable and cut out some branches as well as a function
call.

Microbenchmark results at this point in the stack:

                   │ before.out  │            after-5.out             │
                   │   sec/op    │   sec/op     vs base               │
Malloc8-4            13.52n ± 3%   12.15n ± 2%  -10.13% (p=0.002 n=6)
Malloc16-4           21.49n ± 2%   18.32n ± 4%  -14.75% (p=0.002 n=6)
MallocTypeInfo8-4    27.12n ± 1%   18.64n ± 2%  -31.30% (p=0.002 n=6)
MallocTypeInfo16-4   28.71n ± 3%   21.63n ± 5%  -24.65% (p=0.002 n=6)
geomean              21.81n        17.31n       -20.64%

Change-Id: I5de9ac5089b9eb49bf563af2a74e6dc564420e05
Reviewed-on: https://go-review.googlesource.com/c/go/+/614795
Auto-Submit: Michael Knyszek <mknyszek@google.com>
Reviewed-by: Keith Randall <khr@golang.org>
LUCI-TryBot-Result: Go LUCI <golang-scoped@luci-project-accounts.iam.gserviceaccount.com>
Reviewed-by: Keith Randall <khr@google.com>