[dev.ssa] cmd/compile: optimize phi ops

[dev.ssa] cmd/compile: optimize phi ops

Redo how we keep track of forward references when building SSA.
When the forward reference is resolved, update the Value node
in place.

Improve the phi elimination pass so it can simplify phis of phis.

Give SSA package access to decoded line numbers.  Fix line numbers
for constant booleans.

Change-Id: I3dc9896148d260be2f3dd14cbe5db639ec9fa6b7
Reviewed-on: https://go-review.googlesource.com/18674
Reviewed-by: David Chase <drchase@google.com>
Run-TryBot: Keith Randall <khr@golang.org>