Improve comments for Analysis::IsSelfAssignment.

Change-Id: I34353769f4c65bdfa7205d460671a78f3463a5f2
Reviewed-on: https://skia-review.googlesource.com/c/skia/+/378317
Auto-Submit: John Stiles <johnstiles@google.com>
Commit-Queue: Brian Osman <brianosman@google.com>
Reviewed-by: Brian Osman <brianosman@google.com>

src/sksl/SkSLAnalysis.cpp

@@ -562,16 +562,15 @@ bool Analysis::IsTrivialExpression(const Expression& expr) {
             IsTrivialExpression(*expr.as<IndexExpression>().base()));
 }
 
-/**
- * Returns true if both expression trees are the same. The left side is expected to be an lvalue.
- * This only needs to check for trees that can plausibly terminate in a variable, so some basic
- * candidates like `FloatLiteral` are missing.
- */
 bool Analysis::IsSelfAssignment(const Expression& left, const Expression& right) {
     if (left.kind() != right.kind() || left.type() != right.type()) {
         return false;
     }
 
+    // This isn't a fully exhaustive list of expressions that could be involved in a self-
+    // assignment, particularly when arrays are involved; for instance, `x[y+1] = x[y+1]` isn't
+    // detected because we don't look at BinaryExpressions. Since this is intended to be used for
+    // optimization purposes, handling the common cases is sufficient.
     switch (left.kind()) {
         case Expression::Kind::kIntLiteral:
             return left.as<IntLiteral>().value() == right.as<IntLiteral>().value();

src/sksl/SkSLAnalysis.h

@@ -89,8 +89,7 @@ struct Analysis {
     static bool IsTrivialExpression(const Expression& expr);
 
     // Returns true if both expression trees are the same. The left side is expected to be an
-    // lvalue. This only needs to check for trees that can plausibly terminate in a variable, so
-    // some basic candidates like `FloatLiteral` are missing.
+    // lvalue. Intended for use by the optimizer; won't necessarily catch complex cases.
     static bool IsSelfAssignment(const Expression& left, const Expression& right);
 
     // Is 'expr' a constant-expression, as defined by GLSL 1.0, section 5.10? A constant expression