compiler: revert `for package-scope "a = b; b = x" just set "a = x"`
Revert CL 245098. It caused incorrect initialization ordering.
Adjust the runtime package to work even with the CL reverted.
Original description of CL 245098:
This avoids requiring an init function to initialize the variable.
This can only be done if x is a static initializer.
The go1.15rc1 runtime package relies on this optimization.
The package has a variable "var maxSearchAddr = maxOffAddr".
The maxSearchAddr variable is used by code that runs before package
initialization is complete.
For golang/go#51913
Reviewed-on: https://go-review.googlesource.com/c/gofrontend/+/395994