[NewOptimizer] Union Split during Inlining

This adds union splitting optimizations to the inliner.
This works a bit differently from the old optimizer, which
did union splitting on the bail out path. Instead, we try
to run the full inliner on any union split case that's a
dispatch tuple (and record what to do in that case). Doing
that allows an effective resolution of #23338, though this
PR doesn't quite do that yet, since some further (minor) fixes
are needed.

base/compiler/compiler.jl

@@ -101,15 +101,6 @@ using .Sort
 # compiler #
 ############
 
-inlining_enabled() = (JLOptions().can_inline == 1)
-coverage_enabled() = (JLOptions().code_coverage != 0)
-function inbounds_option()
-    opt_check_bounds = JLOptions().check_bounds
-    opt_check_bounds == 0 && return :default
-    opt_check_bounds == 1 && return :on
-    return :off
-end
-
 include("compiler/utilities.jl")
 include("compiler/validation.jl")
 

base/compiler/ssair/domtree.jl

@@ -23,6 +23,8 @@ function dominates(domtree::DomTree, bb1::Int, bb2::Int)
     return bb1 == bb2
 end
 
+bb_unreachable(domtree::DomTree, bb::Int) = bb != 1 && domtree.nodes[bb].level == 1
+
 function update_level!(domtree::Vector{DomTreeNode}, node::Int, level::Int)
     domtree[node] = DomTreeNode(level, domtree[node].children)
     foreach(domtree[node].children) do child

base/compiler/ssair/driver.jl

@@ -169,6 +169,7 @@ function run_passes(ci::CodeInfo, nargs::Int, linetable::Vector{LineInfoNode}, s
     @timeit "compact 2" ir = compact!(ir)
     @timeit "type lift" ir = type_lift_pass!(ir)
     @timeit "compact 3" ir = compact!(ir)
-    #@timeit "verify 3" (verify_ir(ir); verify_linetable(linetable))
+    #@Base.show ir
+    @timeit "verify 3" (verify_ir(ir); verify_linetable(linetable))
     return ir
 end