cmd/compile: stack allocate variable-sized makeslice

cmd/compile: stack allocate variable-sized makeslice

Instead of always allocating variable-sized "make" calls on the heap,
allocate a small, constant-sized array on the stack and use that array
as the backing store if it is big enough.

Requires the result of the "make" doesn't escape.

  if cap <= K {
      var arr [K]E
      slice = arr[:len:cap]
  } else {
      slice = makeslice(E, len, cap)
  }

Pretty conservatively for now, K = 32/sizeof(E). The slice header is
already 24 bytes, so wasting 32 bytes of stack if the requested size
is too big isn't that bad. Larger would waste more stack space but
maybe avoid more allocations.

This CL also requires the element type be pointer-free.  Maybe we
could relax that at some point, but it is hard. If the element type
has pointers we can get heap->stack pointers (in the case where the
requested size is too big and the slice is heap allocated).

Note that this only handles the case of makeslice called directly from
compiler-generated code. It does not handle slices built in the
runtime on behalf of the program (e.g. in growslice). Some of those
are currently handled by passing in a tmpBuf (e.g. concatstrings),
but we could probably do more.

Change-Id: I8378efad527cd00d25948a80b82a68d88fbd93a1
Reviewed-on: https://go-review.googlesource.com/c/go/+/653856
Reviewed-by: Robert Griesemer <gri@google.com>
LUCI-TryBot-Result: Go LUCI <golang-scoped@luci-project-accounts.iam.gserviceaccount.com>
Reviewed-by: Cherry Mui <cherryyz@google.com>