irinterp: Allow setting all IR flags (#48993)

Currently, `IR_FLAG_NOTHROW` is the only flag that irinterp is allowed to
set on statements, under the assumption that in order for a call to
be irinterp-eligible, it must have been proven `:foldable`, thus `:effect_free`,
and thus `IR_FLAG_EFFECT_FREE` was assumed to have been set. That reasoning
was sound at the time this code was written, but have since introduced
`EFFECT_FREE_IF_INACCESSIBLEMEMONLY`, which breaks the reasoning that
an `:effect_free` inference for the whole function implies the flag on
every statement. As a result, we were failing to DCE otherwise dead
statements if the IR came from irinterp.

base/compiler/ssair/irinterp.jl

@@ -238,12 +238,9 @@ function reprocess_instruction!(interp::AbstractInterpreter,
  head = inst.head
  if head === :call || head === :foreigncall || head === :new || head === :splatnew
  (; rt, effects) = abstract_eval_statement_expr(interp, inst, nothing, ir, irsv.mi)
- # All other effects already guaranteed effect free by construction
- if is_nothrow(effects)
- ir.stmts[idx][:flag] |= IR_FLAG_NOTHROW
- if isa(rt, Const) && is_inlineable_constant(rt.val)
- ir.stmts[idx][:inst] = quoted(rt.val)
- end
+ ir.stmts[idx][:flag] |= flags_for_effects(effects)
+ if is_foldable(effects) && isa(rt, Const) && is_inlineable_constant(rt.val)
+ ir.stmts[idx][:inst] = quoted(rt.val)
  end
  elseif head === :invoke
  mi′ = inst.args[1]::MethodInstance