fix pointerarith_tfunc for Const ptr (#60011)

fixes #60009

(cherry picked from commit 97f880a)

Author: oscardssmith

Compiler/src/tfuncs.jl

@@ -713,7 +713,7 @@ function pointer_eltype(@nospecialize(ptr))
 end
 
 @nospecs function pointerarith_tfunc(𝕃::AbstractLattice, ptr, offset)
-    return ptr
+    return widenconst(ptr)
 end
 @nospecs function pointerref_tfunc(𝕃::AbstractLattice, a, i, align)
     return pointer_eltype(a)

Compiler/test/effects.jl

@@ -1478,3 +1478,10 @@ let effects = Base.infer_effects((Core.SimpleVector,Int); optimize=false) do sve
 end
 
 @test Compiler.is_nothrow(Base.infer_effects(length, (Core.SimpleVector,)))
+
+
+# https://github.com/JuliaLang/julia/issues/60009
+function null_offset(offset)
+    Ptr{UInt8}(C_NULL) + offset
+end
+@test null_offset(Int(100)) == Ptr{UInt8}(UInt(100))