cmd/compile: run deadcode before nilcheck for better statement relocation

Nilcheck would move statements from NilCheck values to others that
turned out were already dead, which leads to lost statements.  Better
to eliminate the dead code first.

One "error" is removed from test/prove.go because the code is
actually dead, and the additional deadcode pass removes it before
prove can run.

Change-Id: If75926ca1acbb59c7ab9c8ef14d60a02a0a94f8b
Reviewed-on: https://go-review.googlesource.com/c/go/+/198479
Run-TryBot: David Chase <drchase@google.com>
TryBot-Result: Gobot Gobot <gobot@golang.org>
Reviewed-by: Jeremy Faller <jeremy@golang.org>

Author: dr2chase

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

@@ -409,6 +409,7 @@ var passes = [...]pass{
 	{name: "opt deadcode", fn: deadcode, required: true}, // remove any blocks orphaned during opt
 	{name: "generic cse", fn: cse},
 	{name: "phiopt", fn: phiopt},
+	{name: "gcse deadcode", fn: deadcode, required: true}, // clean out after cse and phiopt
 	{name: "nilcheckelim", fn: nilcheckelim},
 	{name: "prove", fn: prove},
 	{name: "fuse plain", fn: fusePlain},

test/prove.go

@@ -507,7 +507,7 @@ func sm1(b []int, x int) {
 	useSlice(b[2:8]) // ERROR "Proved slicemask not needed$"
 	// Test non-constant argument with known limits.
 	if cap(b) > 10 {
-		useSlice(b[2:]) // ERROR "Proved slicemask not needed$"
+		useSlice(b[2:])
 	}
 }