A statement like

  foo = bar + qux

might compile to

  AX := AX + BX

resulting in a regkill for AX before this instruction.
The buggy behavior is to kill AX "at" this instruction,
before it has executed.  (Code generation of no-instruction
values like RegKills applies their effects at the
next actual instruction emitted).

However, bar is still associated with AX until after the
instruction executes, so the effect of the regkill must
occur at the boundary between this instruction and the
next.  Similarly, the new value bound to AX is not visible
until this instruction executes (and in the case of values
that require multiple instructions in code generation, until
all of them have executed).

The ranges are adjusted so that a value's start occurs
at the next following instruction after its evaluation,
and the end occurs after (execution of) the first
instruction following the end of the lifetime as a value.

(Notice the asymmetry; the entire value must be finished
before it is visible, but execution of a single instruction
invalidates.  However, the value *is* visible before that
next instruction executes).

The test was adjusted to make it insensitive to the result
numbering for variables printed by gdb, since that is not
relevant to the test and makes the differences introduced
by small changes larger than necessary/useful.

The test was also improved to present variable probes
more intuitively, and also to allow explicit indication
of "this variable was optimized out"

Change-Id: I39453eead8399e6bb05ebd957289b112d1100c0e
Reviewed-on: https://go-review.googlesource.com/74090
Run-TryBot: David Chase <drchase@google.com>
Reviewed-by: Cherry Zhang <cherryyz@google.com>