go.tools/go/ssa/interp: add no-op intrinsic sync.runtime_registerPoolCleanup
go.tools/go/ssa/interp: add no-op intrinsic sync.runtime_registerPoolCleanup

Also, compute correct index of struct field sync.Pool.New, which just changed.

LGTM=gri
R=gri
CC=golang-codereviews
https://codereview.appspot.com/87780043

karmakaze/go.tools,karmakaze/go.tools,chenjianlong/go.tools,karmakaze/go.tools,chenjianlong/go.tools,karmakaze/go.tools,chenjianlong/go.tools,karmakaze/go.tools