Fix regression in generic_bitcast with Union{} arguments.

[maleadt]

#47170 introduced a regression with bitcasts of error values (i.e. ::Union{}), resulting in a failed assertion/segfault:

julia> f(x) = Core.bitcast(UInt64, x)
f (generic function with 1 method)

julia> code_llvm(f, Type{Union{}})
julia-debug: /home/tim/Julia/src/julia/src/intrinsics.cpp:536: jl_cgval_t generic_bitcast(jl_codectx_t&, const jl_cgval_t*): Assertion `!v.isghost' failed.

[730343] signal (6): Aborted
in expression starting at REPL[2]:1
unknown function (ip: 0x7fb345c8c64c)
gsignal at /usr/lib/libc.so.6 (unknown line)
abort at /usr/lib/libc.so.6 (unknown line)
unknown function (ip: 0x7fb345c2645b)
__assert_fail at /usr/lib/libc.so.6 (unknown line)
generic_bitcast at /home/tim/Julia/src/julia/src/intrinsics.cpp:536

This manifested on PkgEval in the testing of ANOVAapprox.jl (as well as Gtk.jl, and probably others), https://s3.amazonaws.com/julialang-reports/nanosoldier/pkgeval/by_date/2022-11/15/ANOVAapprox.primary.log, which contains code like:

struct Foo
    bar::Int32
end

function Base.getproperty(obj::Foo, field::Symbol)
    if field == :whatever
        unsafe_wrap(Vector{Float64}, ptr, obj.bar)
    elseif field == :return
        ccall(:jl_breakpoint, Int64, ())
    else
        getfield(obj, field)
    end
end

Compiling this results in essentially:

%4 = Main.getfield(_2, :bar)::Int32
%32 = π (%4, Int64)
%40 = bitcast(UInt64, %32)

... where inference thinks that %32 produces a Int64, but during codegen it turns out that π (%4::Int32, Int64) produces Union{} (as determined by convert_julia_type) which then crashes bitcast as seen above.

[maleadt]

Rebased and addressed review comments.

[maleadt]

CI failures unrelated; win32 seems broken on master too.