cmd/compile: invalidate CFG when fuseIf triggers

The compiler appears to have a latent bug:
fusePlain calls invalidateCFG when it changes block structure,
but fuseIf does not.

Fix this by hoisting the call to invalidateCFG to the top level.

Change-Id: Ic960fb3ac963b15b4a225aad84863d58efa954e6
Reviewed-on: https://go-review.googlesource.com/c/go/+/177198
Run-TryBot: Josh Bleecher Snyder <josharian@gmail.com>
TryBot-Result: Gobot Gobot <gobot@golang.org>
Reviewed-by: Keith Randall <khr@golang.org>

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

@@ -36,6 +36,9 @@ func fuse(f *Func, typ fuseType) {
 				changed = fuseBlockPlain(b) || changed
 			}
 		}
+		if changed {
+			f.invalidateCFG()
+		}
 	}
 }
 
@@ -207,7 +210,6 @@ func fuseBlockPlain(b *Block) bool {
 	if f.Entry == b {
 		f.Entry = c
 	}
-	f.invalidateCFG()
 
 	// trash b, just in case
 	b.Kind = BlockInvalid