runtime: minor string/rune optimizations

runtime: minor string/rune optimizations

Eliminate a spill in concatstrings.
Provide bounds elim hints in runetochar.
No significant benchmark movement.

Before:
"".runetochar t=1 size=412 args=0x28 locals=0x0
"".concatstrings t=1 size=736 args=0x30 locals=0x98

After:
"".runetochar t=1 size=337 args=0x28 locals=0x0
"".concatstrings t=1 size=711 args=0x30 locals=0x90

Change-Id: Icce646976cb20a223163b7e72a54761193ac17e3
Reviewed-on: https://go-review.googlesource.com/27460
Run-TryBot: Josh Bleecher Snyder <josharian@gmail.com>
Reviewed-by: Martin Möhrmann <martisch@uos.de>
TryBot-Result: Gobot Gobot <gobot@golang.org>