cmd/compile: accept literals in samesafeexpr

cmd/compile: accept literals in samesafeexpr

This only triggers a few places in the stdlib,
but it helps a lot when it does.

Before:

runtime.mapassign1 t=1 size=2400 args=0x20 locals=0xe0

After:

runtime.mapassign1 t=1 size=2352 args=0x20 locals=0xd8

name           old time/op  new time/op  delta
MapPop100-8    19.8µs ±11%  18.4µs ± 9%  -7.16%  (p=0.000 n=20+19)
MapPop1000-8    367µs ±17%   335µs ±11%  -8.63%  (p=0.000 n=19+19)
MapPop10000-8  7.29ms ±15%  6.86ms ±12%  -5.84%  (p=0.020 n=20+20)

Change-Id: I9faf32f95a6ba6a6d5d0818eab32cc271e01d10a
Reviewed-on: https://go-review.googlesource.com/26666
Run-TryBot: Josh Bleecher Snyder <josharian@gmail.com>
TryBot-Result: Gobot Gobot <gobot@golang.org>
Reviewed-by: Matthew Dempsky <mdempsky@google.com>