Fix assert due to unreachable discard (microsoft#7289)

When emitting discard in an unreachable code context (e.g. after an
infinite loop), DXC would assert (if asserts enabled), or trigger a
UBSAN failure because the discard instruction would have no parent. When
an infinite loop is emitted during CodeGen, the InsertPt is cleared,
thus subsequent discard instructions would be created, but no parent
set. We skip emitting discard in this case, which follows the same
pattern as is done for EmitIfStmt, and EmitSwitchStmt.

Author: amaiorano

tools/clang/lib/CodeGen/CGStmt.cpp

@@ -525,6 +525,10 @@ void CodeGenFunction::EmitGotoStmt(const GotoStmt &S) {
 
 // HLSL Change Begins.
 void CodeGenFunction::EmitDiscardStmt(const DiscardStmt &S) {
+  // Skip unreachable discard.
+  if (!HaveInsertPoint())
+    return;
+
   CGM.getHLSLRuntime().EmitHLSLDiscard(*this);
 }
 // HLSL Change Ends.

tools/clang/test/DXC/FinishCodeGen/unreachable-discard.hlsl

@@ -0,0 +1,21 @@
+// RUN: %dxc /T ps_6_5 -fcgl %s | FileCheck %s
+
+// Compiling this HLSL would trigger an assertion:
+//    While deleting: void (i32, float)* %dx.hl.op..void (i32, float)
+//    Use still stuck around after Def is destroyed:  call void @"dx.hl.op..void (i32, float)"(i32 120, float -1.000000e+00), !dbg <0x503000001cc8>
+//    Error: assert(use_empty() && "Uses remain when a value is destroyed!")
+//    File: <snip>/src/external/DirectXShaderCompiler/lib/IR/Value.cpp(83)
+//
+// Bug was fixed in CodeGenFunction::EmitDiscardStmt by skipping the emission of
+// an unreachable discard.
+
+// CHECK:      define void @main()
+// CHECK:      br label %
+// CHECK-NOT:  call void @"dx.hl.op..void (i32, float)"
+// CHECK:      ret void
+
+void main() {
+  while (true) {
+  }
+  discard;
+}