* Shift/rotate by constant doesn't have to stop subprop. (also in 8g)
* Remove redundant MOVLQZX instructions.
* An attempt at issuing loads early.
  Good for 0.5% on a good day, might not be worth keeping.
  Need to understand more about whether the x86
  looks ahead to what loads might be coming up.

R=ken2, ken
CC=golang-dev
https://golang.org/cl/6203091