runtime: use `tophash == emptyRest` to decrease search times in `mapaccess1_faststr` and `mapaccess2_faststr`

[yuanjize]

Proposal Details

In function mapaccess2_faststr and mapaccess2_faststr , I think we can add follow code in dohash block to decrease the search in bucket & over flow buckets.

   if b.tophash[i] == emptyRest {
       return unsafe.Pointer(&zeroVal[0])
   }

Position:

go/src/runtime/map_faststr_noswiss.go

Line 98 in 5276107

k := (*stringStruct)(kptr)

I am pleasure to change it if you think it is good.

[yuanjize]

@mknyszek

[randall77]

Seems reasonable. We're switching over to Swiss tables very soon (#54766), so it might not be worth doing as this code is about to be obsolete.
It will need a benchmark.

[mknyszek]

CC @golang/runtime