This CL makes the bitmaps a little more precise about variables
that have their address taken but for which the address does not
escape to the heap, so that the variables are kept in the stack frame
rather than allocated on the heap.

The code before this CL handled these variables by treating every
return statement as using every such variable and depending on
liveness analysis to essentially treat the variable as live during the
entire function. That approach has false positives and (worse) false
negatives. That is, it's both sloppy and buggy:

        func f(b1, b2 bool) {	// x live here! (sloppy)
                if b2 {
                        print(0) // x live here! (sloppy)
                        return
                }
                var z **int
                x := new(int)
                *x = 42
                z = &x
                print(**z) // x live here (conservative)
                if b2 {
                        print(1) // x live here (conservative)
                        return
                }
                for {
                        print(**z) // x not live here (buggy)
                }
        }

The first two liveness annotations (marked sloppy) are clearly
wrong: x cannot be live if it has not yet been declared.

The last liveness annotation (marked buggy) is also wrong:
x is live here as *z, but because there is no return statement
reachable from this point in the code, the analysis treats x as dead.

This CL changes the liveness calculation to mark such variables
live exactly at points in the code reachable from the variable
declaration. This keeps the conservative decisions but fixes
the sloppy and buggy ones.

The CL also detects ambiguously live variables, those that are
being marked live but may not actually have been initialized,
such as in this example:

        func f(b1 bool) {
                var z **int
                if b1 {
                        x := new(int)
                        *x = 42
                        z = &x
                } else {
                        y := new(int)
                        *y = 54
                        z = &y
                }
                print(**z) // x, y live here (conservative)
        }

Since the print statement is reachable from the declaration of x,
x must conservatively be marked live. The same goes for y.
Although both x and y are marked live at the print statement,
clearly only one of them has been initialized. They are both
"ambiguously live".

These ambiguously live variables cause problems for garbage
collection: the collector cannot ignore them but also cannot
depend on them to be initialized to valid pointer values.

Ambiguously live variables do not come up too often in real code,
but recent changes to the way map and interface runtime functions
are invoked has created a large number of ambiguously live
compiler-generated temporary variables. The next CL will adjust
the analysis to understand these temporaries better, to make
ambiguously live variables fairly rare.

Once ambiguously live variables are rare enough, another CL will
introduce code at the beginning of a function to zero those
slots on the stack. At that point the garbage collector and the
stack copying routines will be able to depend on the guarantee that
if a slot is marked as live in a liveness bitmap, it is initialized.

R=khr
CC=golang-codereviews, iant
https://golang.org/cl/51810043