cmd/6g: avoid taking the address of slices unnecessarily.
cmd/6g: avoid taking the address of slices unnecessarily.

The main case where it happens is when evaluating &s[i] without
bounds checking, which usually happens during range loops (i=0).

This allows registerization of the corresponding variables,
saving 16 bytes of stack frame for each such range loop and a
LEAQ instruction.

R=golang-dev, rsc, dave
CC=golang-dev, remy
http://codereview.appspot.com/6497073

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