When a slice composite literal is sparse, initialize it dynamically
instead of statically.
s := []int{5:5, 20:20}
To initialize the backing store for s, use 2 constant writes instead
of copying from a static array with 21 entries.
This CL also fixes pathologies in the compiler when the slice is
*very* sparse.
Fixes#23780
Change-Id: Iae95c6e6f6a0e2994675cbc750d7a4dd6436b13b
Reviewed-on: https://go-review.googlesource.com/c/151319
Run-TryBot: Keith Randall <khr@golang.org>
Reviewed-by: Robert Griesemer <gri@golang.org>
TryBot-Result: Gobot Gobot <gobot@golang.org>
