cmd/compile,runtime: pass elem type to {make,grow}slice

cmd/compile,runtime: pass elem type to {make,grow}slice

No point in passing the slice type to these functions.
All they need is the element type.  One less indirection,
maybe a few less []T type descriptors in the binary.

Change-Id: Ib0b83b5f14ca21d995ecc199ce8ac00c4eb375e6
Reviewed-on: https://go-review.googlesource.com/22275
Reviewed-by: Josh Bleecher Snyder <josharian@gmail.com>