Handle gc_preserve in lifetime marker insertion

[yuyichao]

In this case, the lifetime is not just determined by the instructions we see anymore.
We need to extend the use list based on domination information before deciding which block has no use of the allocation.

[yuyichao]

The issue can be seen here,

julia> function f()
           ref0 = Ref{UInt32}(0)
           Base.@gc_preserve ref0 ccall(:jl_breakpoint, Void, (Ref{Ptr{Void}},),
                                        Ref{Ptr{Void}}(pointer_from_objref(ref0)))
       end
f (generic function with 1 method)

julia> @code_llvm f()

; Function f
; Location: REPL[1]
define %jl_value_t addrspace(10)* @japi1_f_64085(%jl_value_t addrspace(10)*, %jl_value_t addrspace(10)**, i32) #0 {
L14:
  %3 = alloca i64, align 8
  %4 = alloca i32, align 4
  %5 = alloca %jl_value_t addrspace(10)**, align 8
  store volatile %jl_value_t addrspace(10)** %1, %jl_value_t addrspace(10)*** %5, align 8
  %6 = bitcast i32* %4 to i8*
  %7 = bitcast i64* %3 to i8*
  call void @llvm.lifetime.start.p0i8(i64 4, i8* %6)
; Location: REPL[1]:2
  store i32 0, i32* %4, align 4
  %8 = ptrtoint i32* %4 to i64
  call void @llvm.lifetime.start.p0i8(i64 8, i8* %7)
; Location: REPL[1]:3
; Function macro expansion; {
; Location: pointer.jl:169
  store i64 %8, i64* %3, align 8
  call void inttoptr (i64 139985854009568 to void (i8*)*)(i8* %7)
;}
  ret %jl_value_t addrspace(10)* addrspacecast (%jl_value_t* inttoptr (i64 139985567744008 to %jl_value_t*) to %jl_value_t addrspace(10)*)
}

The allocation optimization incorrectly determined that the allocation after the gc_preserve_begin, which is the last visible use of the first allocation, marks the end of the lifetime. With this patch, we get

; Function f
; Location: REPL[1]
define %jl_value_t addrspace(10)* @japi1_f_64028(%jl_value_t addrspace(10)*, %jl_value_t addrspace(10)**, i32) #0 {
L14:
  %3 = alloca i64, align 8
  %4 = alloca i32, align 4
  %5 = alloca %jl_value_t addrspace(10)**, align 8
  store volatile %jl_value_t addrspace(10)** %1, %jl_value_t addrspace(10)*** %5, align 8
  %6 = bitcast i64* %3 to i8*
; Location: REPL[1]:2
  store i32 0, i32* %4, align 4
  %7 = ptrtoint i32* %4 to i64
  call void @llvm.lifetime.start.p0i8(i64 8, i8* %6)
; Location: REPL[1]:3
; Function macro expansion; {
; Location: pointer.jl:169
  store i64 %7, i64* %3, align 8
  call void inttoptr (i64 139892293033712 to void (i8*)*)(i8* %6)
;}
  ret %jl_value_t addrspace(10)* addrspacecast (%jl_value_t* inttoptr (i64 139892005834760 to %jl_value_t*) to %jl_value_t addrspace(10)*)
}

instead.

[yuyichao]

Actually it turns out that implementing the correct version is faster than waiting for the CI... So here it is...