cmd/compile: eliminate write barriers when writing non-heap ptrs

cmd/compile: eliminate write barriers when writing non-heap ptrs

We don't need a write barrier if:
1) The location we're writing to doesn't hold a heap pointer, and
2) The value we're writing isn't a heap pointer.

The freshly returned value from runtime.newobject satisfies (1).
Pointers to globals, and the contents of the read-only data section satisfy (2).

This is particularly helpful for code like:
p := []string{"abc", "def", "ghi"}

Where the compiler generates:
   a := new([3]string)
   move(a, statictmp_)  // eliminates write barriers here
   p := a[:]

For big slice literals, this makes the code a smaller and faster to
compile.

Update #13554. Reduces the compile time by ~10% and RSS by ~30%.

Change-Id: Icab81db7591c8777f68e5d528abd48c7e44c87eb
Reviewed-on: https://go-review.googlesource.com/c/151498
Run-TryBot: Keith Randall <khr@golang.org>
TryBot-Result: Gobot Gobot <gobot@golang.org>
Reviewed-by: Austin Clements <austin@google.com>
Reviewed-by: Cherry Zhang <cherryyz@google.com>