runtime: use multiplication with overflow check for newarray
This improves performance for e.g. maps with a bucket size
(key+value*8 bytes) larger than 32 bytes and removes loading
a value from the maxElems array for smaller bucket sizes.
name old time/op new time/op delta
MakeMap/[Byte]Byte 95.5ns ± 1% 94.7ns ± 1% -0.78% (p=0.013 n=9+9)
MakeMap/[Int]Int 128ns ± 0% 121ns ± 2% -5.63% (p=0.000 n=6+10)
Updates #21588
Change-Id: I7d9eb7d49150c399c15dcab675e24bc97ff97852
Reviewed-on: https://go-review.googlesource.com/c/143997 Reviewed-by: Keith Randall <khr@golang.org>