[dev.ssa] cmd/compile: make failed nil checks panic

Introduce pseudo-ops PanicMem and LoweredPanicMem.
PanicMem could be rewritten directly into MOVL
during lowering, but then we couldn't log nil checks.

With this change, runnable nil check tests pass:

GOSSAPKG=main go run run.go -- nil*.go

Compiler output nil check tests fail:

GOSSAPKG=p go run run.go -- nil*.go

This is due to several factors:

* SSA has improved elimination of unnecessary nil checks.
* SSA is missing elimination of implicit nil checks.
* SSA is missing extra logging about why nil checks were removed.

I'm not sure how best to resolve these failures,
particularly in a world in which the two backends
will live side by side for some time.
For now, punt on the problem.

Change-Id: Ib2ca6824551671f92e0e1800b036f5ca0905e2a3
Reviewed-on: https://go-review.googlesource.com/13474
Reviewed-by: Keith Randall <khr@golang.org>

src/cmd/compile/internal/ssa/gen/AMD64Ops.go

@@ -287,6 +287,9 @@ func init() {
 		// Rewrites will convert this to (SETG (CMPQ b a)).
 		// InvertFlags is a pseudo-op which can't appear in assembly output.
 		{name: "InvertFlags"}, // reverse direction of arg0
+
+		// LoweredPanicNilCheck is a pseudo-op.
+		{name: "LoweredPanicNilCheck"},
 	}
 
 	var AMD64blocks = []blockData{