[Clang][AArch64][SVE] Allow write to SVE vector elements using the subscript operator

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 `insertvector`.

Change-Id: I46d0333d8ed8508cd9cd23e02dd1c2d48fb74cd2

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

@@ -5227,7 +5227,7 @@ 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 +88,13 @@ 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;
+}