runtime,cmd/compile: pass strings and slices to convT2{E,I} by value

When we pass these types by reference, we usually have to allocate
temporaries on the stack, initialize them, then pass their address
to the conversion functions. It's simpler to pass these types
directly by value.

This particularly applies to conversions needed for fmt.Printf
(to interface{} for constructing a [...]interface{}).

func f(a, b, c string) {
     fmt.Printf("%s %s\n", a, b)
     fmt.Printf("%s %s\n", b, c)
}

This function's stack frame shrinks from 200 to 136 bytes, and
its code shrinks from 535 to 453 bytes.

The go binary shrinks 0.3%.

Update #24286

Aside: for this function f, we don't really need to allocate
temporaries for the convT2E function. We could use the address
of a, b, and c directly. That might get similar (or maybe better?)
improvements. I investigated a bit, but it seemed complicated
to do it safely. This change was much easier.

Change-Id: I78cbe51b501fb41e1e324ce4203f0de56a1db82d
Reviewed-on: https://go-review.googlesource.com/c/135377


Run-TryBot: Keith Randall <khr@golang.org>
TryBot-Result: Gobot Gobot <gobot@golang.org>
Reviewed-by: Josh Bleecher Snyder <josharian@gmail.com>