* Avoid calculating insertk until needed.
* Avoid a pointer into b.tophash and just track the insertion index.
  This avoids b.tophash being marked as escaping to heap.
* Calculate val only once at the end of the mapassign functions.

Function sizes decrease slightly, e.g. for mapassign_faststr:
before "".mapassign_faststr STEXT size=1166 args=0x28 locals=0x78
after  "".mapassign_faststr STEXT size=1080 args=0x28 locals=0x68

name                     old time/op  new time/op  delta
MapAssign/Int32/256-4    19.4ns ± 4%  19.5ns ±11%     ~     (p=0.973 n=20+20)
MapAssign/Int32/65536-4  32.5ns ± 2%  32.4ns ± 3%     ~     (p=0.078 n=20+19)
MapAssign/Int64/256-4    20.3ns ± 6%  17.6ns ± 5%  -13.01%  (p=0.000 n=20+20)
MapAssign/Int64/65536-4  33.3ns ± 2%  33.3ns ± 1%     ~     (p=0.444 n=20+20)
MapAssign/Str/256-4      22.3ns ± 3%  22.4ns ± 3%     ~     (p=0.343 n=20+20)
MapAssign/Str/65536-4    44.9ns ± 1%  43.9ns ± 1%   -2.39%  (p=0.000 n=20+19)

Change-Id: I2627bb8a961d366d9473b5922fa129176319eb22
Reviewed-on: https://go-review.googlesource.com/74870
Run-TryBot: Martin Möhrmann <moehrmann@google.com>
TryBot-Result: Gobot Gobot <gobot@golang.org>
Reviewed-by: Keith Randall <khr@golang.org>