[Clang][AArch64][SVE] Allow write to SVE vector elements using the subscript operator (#91965)

The patch at https://reviews.llvm.org/D122732 introduced using the array
subscript operator for SVE vectors, however it also causes an ICE when
the subscripting expression is used as an lvalue.

This patches fixes the error. Lvalue subscripting expressions are
emitted as LLVM IR `insertelement`.

Author: momchil-velikov

clang/lib/CodeGen/CGExpr.cpp

@@ -4180,7 +4180,7 @@ LValue CodeGenFunction::EmitArraySubscriptExpr(const ArraySubscriptExpr *E,
 
   // If the base is a vector type, then we are forming a vector element lvalue
   // with this subscript.
-  if (E->getBase()->getType()->isVectorType() &&
+  if (E->getBase()->getType()->isSubscriptableVectorType() &&
       !isa<ExtVectorElementExpr>(E->getBase())) {
     // Emit the vector as an lvalue to get its address.
     LValue LHS = EmitLValue(E->getBase());

clang/lib/Sema/SemaExpr.cpp

@@ -5185,7 +5185,7 @@ Sema::CreateBuiltinArraySubscriptExpr(Expr *Base, SourceLocation LLoc,
   }
 
   // Perform default conversions.
-  if (!LHSExp->getType()->getAs<VectorType>()) {
+  if (!LHSExp->getType()->isSubscriptableVectorType()) {
     ExprResult Result = DefaultFunctionArrayLvalueConversion(LHSExp);
     if (Result.isInvalid())
       return ExprError();

clang/test/CodeGen/aarch64-sve-vector-subscript-ops.c

@@ -88,3 +88,25 @@ float subscript_float32(svfloat32_t a, size_t b) {
 double subscript_float64(svfloat64_t a, size_t b) {
   return a[b];
 }
+
+// CHECK-LABEL: @subscript_write_float32(
+// CHECK-NEXT:  entry:
+// CHECK-NEXT:    [[VECINS:%.*]] = insertelement <vscale x 4 x float> [[A:%.*]], float 1.000000e+00, i64 [[B:%.*]]
+// CHECK-NEXT:    ret <vscale x 4 x float> [[VECINS]]
+//
+svfloat32_t subscript_write_float32(svfloat32_t a, size_t b) {
+  a[b] = 1.0f;
+  return a;
+}
+
+// CHECK-LABEL: @subscript_read_write_float32(
+// CHECK-NEXT:  entry:
+// CHECK-NEXT:    [[VECEXT:%.*]] = extractelement <vscale x 4 x float> [[A:%.*]], i64 [[B:%.*]]
+// CHECK-NEXT:    [[ADD:%.*]] = fadd float [[VECEXT]], 1.000000e+00
+// CHECK-NEXT:    [[VECINS:%.*]] = insertelement <vscale x 4 x float> [[A]], float [[ADD]], i64 [[B]]
+// CHECK-NEXT:    ret <vscale x 4 x float> [[VECINS]]
+//
+svfloat32_t subscript_read_write_float32(svfloat32_t a, size_t b) {
+  a[b] += 1.0f;
+  return a;
+}