Instead of comparing if the number of elements will
not fit into memory check if the memory size of the
slices backing memory is higher then the memory limit.

This avoids a division or maxElems lookup.

With et.size > 0:
   uintptr(newcap)                > maxSliceCap(et.size)
-> uintptr(int(capmem / et.size)) > _MaxMem  /  et.size
->             capmem / et.size   > _MaxMem  /  et.size
->             capmem             > _MaxMem

Note that due to integer division from capmem > _MaxMem
it does not follow that uintptr(newcap) > maxSliceCap(et.size).

Consolidated runtime GrowSlice benchmarks by using sub-benchmarks and
added more struct sizes to show performance improvement when division
is avoided for element sizes larger than 32 bytes.

AMD64:
GrowSlice/Byte       38.9ns ± 2%  38.9ns ± 1%    ~     (p=0.974 n=20+20)
GrowSlice/Int        58.3ns ± 3%  58.0ns ± 2%    ~     (p=0.154 n=20+19)
GrowSlice/Ptr        95.7ns ± 2%  95.1ns ± 2%  -0.60%  (p=0.034 n=20+20)
GrowSlice/Struct/24  95.4ns ± 1%  93.9ns ± 1%  -1.54%  (p=0.000 n=19+19)
GrowSlice/Struct/32   110ns ± 1%   108ns ± 1%  -1.76%  (p=0.000 n=19+20)
GrowSlice/Struct/40   138ns ± 1%   128ns ± 1%  -7.09%  (p=0.000 n=20+20)

Change-Id: I1c37857c74ea809da373e668791caffb6a5cbbd3
Reviewed-on: https://go-review.googlesource.com/53471
Run-TryBot: Martin Möhrmann <moehrmann@google.com>
TryBot-Result: Gobot Gobot <gobot@golang.org>
Reviewed-by: Keith Randall <khr@golang.org>