IRShow: use IOContext for PhiNodes/:invoke printing (#43226)

In order to allow `Argument`s to be printed nicely.

> before
```julia
julia> code_typed((Float64,)) do x
           sin(x)
       end
1-element Vector{Any}:
 CodeInfo(
1 ─ %1 = invoke Main.sin(_2::Float64)::Float64
└──      return %1
) => Float64

julia> code_typed((Bool,Any,Any)) do c, x, y
           z = c ? x : y
           z
       end
1-element Vector{Any}:
 CodeInfo(
1 ─      goto #3 if not c
2 ─      goto #4
3 ─      nothing::Nothing
4 ┄ %4 = φ (#2 => _3, #3 => _4)::Any
└──      return %4
) => Any
```

> after
```julia
julia> code_typed((Float64,)) do x
           sin(x)
       end
1-element Vector{Any}:
 CodeInfo(
1 ─ %1 = invoke Main.sin(x::Float64)::Float64
└──      return %1
) => Float64

julia> code_typed((Bool,Any,Any)) do c, x, y
           z = c ? x : y
           z
       end
1-element Vector{Any}:
 CodeInfo(
1 ─      goto #3 if not c
2 ─      goto #4
3 ─      nothing::Nothing
4 ┄ %4 = φ (#2 => x, #3 => y)::Any
└──      return %4
) => Any
```

base/compiler/ssair/show.jl

@@ -47,7 +47,7 @@ function print_stmt(io::IO, idx::Int, @nospecialize(stmt), used::BitSet, maxleng
         # XXX: this is wrong if `sig` is not a concretetype method
         # more correct would be to use `fieldtype(sig, i)`, but that would obscure / discard Varargs information in show
         sig = linfo.specTypes == Tuple ? Core.svec() : Base.unwrap_unionall(linfo.specTypes).parameters::Core.SimpleVector
-        print_arg(i) = sprint() do io
+        print_arg(i) = sprint(; context=io) do io
             show_unquoted(io, stmt.args[i], indent)
             if (i - 1) <= length(sig)
                 print(io, "::", sig[i - 1])
@@ -82,7 +82,7 @@ function show_unquoted_phinode(io::IO, stmt::PhiNode, indent::Int, prefix::Strin
     args = String[let
         e = stmt.edges[i]
         v = !isassigned(stmt.values, i) ? "#undef" :
-            sprint() do io′
+            sprint(; context=io) do io′
                 show_unquoted(io′, stmt.values[i], indent)
             end
         "$prefix$e => $v"