cmd/compile: do not allocate bucket for non-escaping map

For map with hint larger than BUCKETSIZE, makemap ignore allocated
bucket and allocate buckets itself. So do not allocate bucket in
this case, save us the cost of zeroing+assignment to the bucket.

name                            old time/op    new time/op    delta
NewEmptyMap-12                    3.89ns ± 4%    3.88ns ± 2%    ~     (p=0.939 n=19+20)
NewSmallMap-12                    23.3ns ± 3%    23.1ns ± 2%    ~     (p=0.307 n=18+17)
NewEmptyMapHintLessThan8-12       6.43ns ± 3%    6.31ns ± 2%  -1.72%  (p=0.000 n=19+18)
NewEmptyMapHintGreaterThan8-12     159ns ± 2%     150ns ± 1%  -5.79%  (p=0.000 n=20+18)

Benchmark run with commit ab7c1741 reverted, see #38314.

Fixes #20184

Change-Id: Ic021f57454c3a0dd50601d73bbd77b8faf8d93b6
Reviewed-on: https://go-review.googlesource.com/c/go/+/227458


Run-TryBot: Cuong Manh Le <cuong.manhle.vn@gmail.com>
TryBot-Result: Gobot Gobot <gobot@golang.org>
Reviewed-by: Keith Randall <khr@golang.org>