cmd/compile: generalize isfat to handle 1-field structs and 1-element arrays

After CL 192979, it is safe now to optimize isfat slightly to handle
1-field structs and 1-element arrays.

Change-Id: Ie3bc30299abbcef36eee7a0681997cc2f88ed6a3
Reviewed-on: https://go-review.googlesource.com/c/go/+/192980
Run-TryBot: Cuong Manh Le <cuong.manhle.vn@gmail.com>
TryBot-Result: Gobot Gobot <gobot@golang.org>
Reviewed-by: Matthew Dempsky <mdempsky@google.com>

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

@@ -304,7 +304,7 @@ func (lv *Liveness) valueEffects(v *ssa.Value) (int32, liveEffect) {
 	var effect liveEffect
 	// Read is a read, obviously.
 	//
-	// Addr is a read also, as any subseqent holder of the pointer must be able
+	// Addr is a read also, as any subsequent holder of the pointer must be able
 	// to see all the values (including initialization) written so far.
 	// This also prevents a variable from "coming back from the dead" and presenting
 	// stale pointers to the garbage collector. See issue 28445.
@@ -1450,12 +1450,25 @@ func liveness(e *ssafn, f *ssa.Func, pp *Progs) LivenessMap {
 	return lv.livenessMap
 }
 
+// TODO(cuonglm,mdempsky): Revisit after #24416 is fixed.
 func isfat(t *types.Type) bool {
 	if t != nil {
 		switch t.Etype {
-		case TSTRUCT, TARRAY, TSLICE, TSTRING,
+		case TSLICE, TSTRING,
 			TINTER: // maybe remove later
 			return true
+		case TARRAY:
+			// Array of 1 element, check if element is fat
+			if t.NumElem() == 1 {
+				return isfat(t.Elem())
+			}
+			return true
+		case TSTRUCT:
+			// Struct with 1 field, check if field is fat
+			if t.NumFields() == 1 {
+				return isfat(t.Field(0).Type)
+			}
+			return true
 		}
 	}