Consider the following test case:

% cat bug.cc
class base
{
public:
  base() {}
  virtual ~base() {}
  virtual void destroy() = 0;
  virtual void destroy_deallocate() = 0;
};

class bar
{
public:
  bar() : _f(0) {}
  ~bar()
  {
    if ((void *)_f == &_buf)
      _f->destroy();
    else if (_f)
      _f->destroy_deallocate();
  }

  void* _buf;
  base* _f;
};

extern "C" {
unsigned sleep (unsigned int __seconds);
}

extern void foo(bar x);

void baz()
{
  bar x;
  foo(x);
  while (1) sleep(10);
}

% clang --target=x86_64-unknown-unknown -S -o - bug.cc -O2 -fno-exceptions -fno-rtti -mllvm -enable-gvn-hoist=false
        .text
        .file   "bug.cc"
        .globl  _Z3bazv
        .p2align        4, 0x90
        .type   _Z3bazv,@function
_Z3bazv:                                # @_Z3bazv
        .cfi_startproc
# BB#0:                                 # %entry
        pushq   %rbp
.Lcfi0:
        .cfi_def_cfa_offset 16
.Lcfi1:
        .cfi_offset %rbp, -16
        movq    %rsp, %rbp
.Lcfi2:
        .cfi_def_cfa_register %rbp
        pushq   %rbx
        subq    $24, %rsp
.Lcfi3:
        .cfi_offset %rbx, -24
        movq    $0, -16(%rbp)
        leaq    -24(%rbp), %rbx
        movq    %rbx, %rdi
        callq   _Z3foo3bar
        movq    -16(%rbp), %rdi
        cmpq    %rbx, %rdi
        je      .LBB0_1
# BB#2:                                 # %if.else.i
        testq   %rdi, %rdi              # NULL TEST
        je      .LBB0_4
# BB#3:                                 # %if.then4.i
        movq    (%rdi), %rax            # LOAD
        callq   *24(%rax)
        jmp     .LBB0_4
.LBB0_1:                                # %if.then.i
        movq    (%rdi), %rax
        callq   *16(%rax)
        .p2align        4, 0x90
.LBB0_4:                                # %while.cond
                                        # =>This Inner Loop Header: Depth=1
        movl    $10, %edi
        callq   sleep
        jmp     .LBB0_4
.Lfunc_end0:
        .size   _Z3bazv, .Lfunc_end0-_Z3bazv
        .cfi_endproc


        .ident  "clang version 5.0.0 (trunk 301138) (llvm/trunk 301148)"
        .section        ".note.GNU-stack","",@progbits

versus:

% clang --target=x86_64-unknown-unknown -S -o - bug.cc -O2 -fno-exceptions -fno-rtti -mllvm -enable-gvn-hoist=true
        .text
        .file   "bug.cc"
        .globl  _Z3bazv
        .p2align        4, 0x90
        .type   _Z3bazv,@function
_Z3bazv:                                # @_Z3bazv
        .cfi_startproc
# BB#0:                                 # %entry
        pushq   %rbp
.Lcfi0:
        .cfi_def_cfa_offset 16
.Lcfi1:
        .cfi_offset %rbp, -16
        movq    %rsp, %rbp
.Lcfi2:
        .cfi_def_cfa_register %rbp
        pushq   %rbx
        subq    $24, %rsp
.Lcfi3:
        .cfi_offset %rbx, -24
        movq    $0, -16(%rbp)
        leaq    -24(%rbp), %rbx
        movq    %rbx, %rdi
        callq   _Z3foo3bar
        movq    -16(%rbp), %rdi
        cmpq    %rbx, %rdi
        movq    (%rdi), %rax            # CRASH
        je      .LBB0_1
# BB#2:                                 # %if.else.i
        testq   %rdi, %rdi              # NULL TEST
        je      .LBB0_4
# BB#3:                                 # %if.then4.i
        callq   *24(%rax)
        jmp     .LBB0_4
.LBB0_1:                                # %if.then.i
        callq   *16(%rax)
        .p2align        4, 0x90
.LBB0_4:                                # %while.cond
                                        # =>This Inner Loop Header: Depth=1
        movl    $10, %edi
        callq   sleep
        jmp     .LBB0_4
.Lfunc_end0:
        .size   _Z3bazv, .Lfunc_end0-_Z3bazv
        .cfi_endproc


        .ident  "clang version 5.0.0 (trunk 301138) (llvm/trunk 301148)"
        .section        ".note.GNU-stack","",@progbits


See the lines marked "NULL TEST", "LOAD", and "CRASH". GVN Hoist is lifting the load through %rdi in this example across the null test.

You can also see this in the IR, nothing x86 specific here.