[dev.ssa] cmd/compile: be safer about uintptr/unsafe.Pointer conversions

Make sure that when a pointer value is live across a function
call, we save it as a pointer.  (And similarly a uintptr
live across a function call should not be saved as a pointer.)

Add a nasty test case.

This is probably what is preventing the merge from master
to dev.ssa.  Signs point to something like this bug happening
in mallocgc.

Change-Id: Ib23fa1251b8d1c50d82c6a448cb4a4fc28219029
Reviewed-on: https://go-review.googlesource.com/16830
Run-TryBot: Keith Randall <khr@golang.org>
TryBot-Result: Gobot Gobot <gobot@golang.org>
Reviewed-by: David Chase <drchase@google.com>

src/cmd/compile/internal/gc/ssa_test.go

@@ -93,3 +93,5 @@ func TestZero(t *testing.T) { runTest(t, "zero_ssa.go") }
 func TestAddressed(t *testing.T) { runTest(t, "addressed_ssa.go") }
 
 func TestCopy(t *testing.T) { runTest(t, "copy_ssa.go") }
+
+func TestUnsafe(t *testing.T) { runTest(t, "unsafe_ssa.go") }