Code validation and IR verification improvements

[maleadt]

Fixes this code validation error:

❯ julia -g2

julia> function test(a)
           f = Base.Experimental.@opaque(()->nothing)
           return 0
       end
test (generic function with 1 method)

julia> Base.code_ircode(test, (Any,))
WARNING: Encountered invalid lowered code for method test(Any): Core.Compiler.InvalidCodeError(kind="invalid RHS value", meta=Expr(:new_opaque_closure, SSAValue(1), SSAValue(2), Core.Any, Tuple))

This is already covered by our tests; we just don't happen to run with -g2 to enable validation (which is what #52830 is aiming to change).

---

Also fixes this IR verification issue:

    julia: /cache/build/builder-amdci4-3/julialang/julia-master/src/task.c:447: ctx_switch: Assertion `ptls->locks.len == 0' failed.
[957] signal 6 (-6): Aborted
in expression starting at /cache/build/tester-amdci4-8/julialang/julia-master/julia-0819ad94c1/share/julia/test/subtype.jl:1100
gsignal at /lib/x86_64-linux-gnu/libc.so.6 (unknown line)
abort at /lib/x86_64-linux-gnu/libc.so.6 (unknown line)
unknown function (ip: 0x7fd5d2fc240e)
__assert_fail at /lib/x86_64-linux-gnu/libc.so.6 (unknown line)
ctx_switch at /cache/build/builder-amdci4-3/julialang/julia-master/src/task.c:447
ijl_switch at /cache/build/builder-amdci4-3/julialang/julia-master/src/task.c:657
try_yieldto at ./task.jl:934
wait at ./task.jl:1008
uv_write at ./stream.jl:1060
...
display at ./multimedia.jl:254
display at ./multimedia.jl:255 [inlined]
display at ./multimedia.jl:340
jfptr_display_53896.1 at /cache/build/tester-amdci4-8/julialang/julia-master/julia-0819ad94c1/lib/julia/sys.so (unknown line)
_jl_invoke at /cache/build/builder-amdci4-3/julialang/julia-master/src/gf.c:2924 [inlined]
ijl_apply_generic at /cache/build/builder-amdci4-3/julialang/julia-master/src/gf.c:3101
jl_apply at /cache/build/builder-amdci4-3/julialang/julia-master/src/julia.h:2152 [inlined]
jl_f__call_latest at /cache/build/builder-amdci4-3/julialang/julia-master/src/builtins.c:875
#invokelatest#3 at ./essentials.jl:955 [inlined]
invokelatest at ./essentials.jl:952 [inlined]
maybe_show_ir at ./compiler/ssair/verify.jl:5 [inlined]
verify_ir at ./compiler/ssair/verify.jl:18

... by using Core.stdout as an unbuffered version of our libuv output streams.