runtime: apply strength reduction optimizations to generic map routines

[josharian]

CL 57590 applied strength reduction to the key pointers in some mapfast routines. This is a reminder issue to check whether similar optimizations can fruitfully be applied to any of the generic map routines (access, assign, delete, etc.).

cc @martisch

[gopherbot]

Change https://golang.org/cl/59390 mentions this issue: runtime: strength reduce key pointer calculations in generic map functions