[clang] Incorrect IR involving the use of bcopy (#79298)

This patch addresses the issue regarding the call of bcopy function in a
conditional expression.
It is analogous to the already accepted patch which deals with the same
problem, just regarding the bzero function [0].

Here is the testcase which illustrates the issue:

```
void bcopy(const void *, void *, unsigned long);
void foo(void);

void test_bcopy() {
  char dst[20];
  char src[20];
  int _sz = 20, len = 20;
  return (_sz
          ? ((_sz >= len)
             ? bcopy(src, dst, len)
             : foo())
          : bcopy(src, dst, len));
}
```

When processing it with clang, following issue occurs:

Instruction does not dominate all uses!
%arraydecay2 = getelementptr inbounds [20 x i8], ptr %dst, i64 0, i64 0,
!dbg !38
%cond = phi ptr [ %arraydecay2, %cond.end ], [ %arraydecay5,
%cond.false3 ], !dbg !33
fatal error: error in backend: Broken module found, compilation aborted!

This happens because an incorrect phi node is created. It is created
because bcopy function call is lowered to the call of llvm.memmove
intrinsic and function memmove returns void *. Since llvm.memmove is
called in two places in the same return statement, clang creates a phi
node in the final basic block for the return value and that phi node is
incorrect. However, bcopy function should return void in the first
place, so this phi node is unnecessary. This is what this patch
addresses. An appropriate test is also added and no existing tests fail
when applying this patch.

Also, this crash only happens when LLVM is configured with
-DLLVM_ENABLE_ASSERTIONS=On option.

[0] https://reviews.llvm.org/D39746

Author: vojkan99

clang/lib/CodeGen/CGBuiltin.cpp

@@ -4011,7 +4011,7 @@ RValue CodeGenFunction::EmitBuiltinExpr(const GlobalDecl GD, unsigned BuiltinID,
     EmitNonNullArgCheck(RValue::get(Dest.getPointer()), E->getArg(1)->getType(),
                         E->getArg(1)->getExprLoc(), FD, 0);
     Builder.CreateMemMove(Dest, Src, SizeVal, false);
-    return RValue::get(Dest.getPointer());
+    return RValue::get(nullptr);
   }
 
   case Builtin::BImemcpy:

clang/test/CodeGen/builtins.c

@@ -202,6 +202,21 @@ void test_conditional_bzero(void) {
   // CHECK-NOT: phi
 }
 
+// CHECK-LABEL: define{{.*}} void @test_conditional_bcopy
+void test_conditional_bcopy(void) {
+  char dst[20];
+  char src[20];
+  int _sz = 20, len = 20;
+  return (_sz
+          ? ((_sz >= len)
+              ? __builtin_bcopy(src, dst, len)
+              : foo())
+          : __builtin_bcopy(src, dst, len));
+  // CHECK: call void @llvm.memmove
+  // CHECK: call void @llvm.memmove
+  // CHECK-NOT: phi
+}
+
 // CHECK-LABEL: define{{.*}} void @test_float_builtins
 void test_float_builtins(__fp16 *H, float F, double D, long double LD) {
   volatile int res;