Allow removing unused pointerref
This allows the julia-level optimizer to drop unused pointerref.
This matches LLVM in that unused non-volatile loads are allowed to
be dropped, despite them having possibly observable side effects
(segfaults), since those are modeled as undefined. When heavily
optimizing code we frequently saw a lot of left over pointerref
calls resulting from inlining `length(::SimpleVector)`. We have a
special tfunc for that call (that can give us the result as a const),
but inline it anyway and thus end up with a pointerref whose result
is unused that we didn't used to be able to eliminate. E.g.
```
julia> f(T::Type{S}) where {S} = Val(length(T.parameters))
f (generic function with 1 method)
```
Before:
```
julia> @code_typed f(Int)
CodeInfo(
1 ─ %1 = (Base.getfield)(T, :parameters)::SimpleVector
│ %2 = $(Expr(:gc_preserve_begin, :(%1)))
│ %3 = $(Expr(:foreigncall, :(:jl_value_ptr), Ptr{Nothing}, svec(Any), :(:ccall), 1, :(%1)))::Ptr{Nothing}
│ %4 = (Base.bitcast)(Ptr{Int64}, %3)::Ptr{Int64}
│ (Base.pointerref)(%4, 1, 1)::Int64
│ $(Expr(:gc_preserve_end, :(%2)))
└── return $(QuoteNode(Val{0}()))
) => Val{0}
```
After:
```
julia> @code_typed f(Int)
CodeInfo(
1 ─ %1 = (Base.getfield)(T, :parameters)::SimpleVector
│ %2 = $(Expr(:gc_preserve_begin, :(%1)))
│ $(Expr(:foreigncall, :(:jl_value_ptr), Ptr{Nothing}, svec(Any), :(:ccall), 1, :(%1)))::Ptr{Nothing}
│ $(Expr(:gc_preserve_end, :(%2)))
└── return $(QuoteNode(Val{0}()))
) => Val{0}
```
Of course we still have the useless foreigncall. That is outside
the scope of this PR, but I'm hoping to have a general solution
in the future to mark foreigncalls as effect free if unused.
