[mlir][flang] WIP patch for CSE AA callback

[ashermancinelli]

The test flang/test/Lower/array-constructor-2.f90 gave me the most pause because the filecheck tests are for after cfg-conversion. This is me convincing myself that CSE is not doing anything unsafe in this test case:

What changed at the hlfir level?

   %43 = fir.load %13 : !fir.ref<!fir.box<!fir.heap<!fir.array<?xf32>>>>
   %44 = fir.box_addr %43 : (!fir.box<!fir.heap<!fir.array<?xf32>>>) -> !fir.heap<!fir.array<?xf32>>
   %45 = fir.convert %44 : (!fir.heap<!fir.array<?xf32>>) -> i64
   %46 = arith.cmpi ne, %45, %c0_i64 : i64
   fir.if %46 {
-    %51 = fir.load %13 : !fir.ref<!fir.box<!fir.heap<!fir.array<?xf32>>>>
-    %52 = fir.box_addr %51 : (!fir.box<!fir.heap<!fir.array<?xf32>>>) -> !fir.heap<!fir.array<?xf32>>
-    fir.freemem %52 : !fir.heap<!fir.array<?xf32>>
+    fir.freemem %44 : !fir.heap<!fir.array<?xf32>>
     fir.store %10 to %13 : !fir.ref<!fir.box<!fir.heap<!fir.array<?xf32>>>>
   }
   %47 = fir.load %11 : !fir.ref<!fir.box<!fir.heap<!fir.array<?xf32>>>>
   %48 = fir.box_addr %47 : (!fir.box<!fir.heap<!fir.array<?xf32>>>) -> !fir.heap<!fir.array<?xf32>>
   %49 = fir.convert %48 : (!fir.heap<!fir.array<?xf32>>) -> i64
   %50 = arith.cmpi ne, %49, %c0_i64 : i64
   fir.if %50 {
-    %51 = fir.load %11 : !fir.ref<!fir.box<!fir.heap<!fir.array<?xf32>>>>
-    %52 = fir.box_addr %51 : (!fir.box<!fir.heap<!fir.array<?xf32>>>) -> !fir.heap<!fir.array<?xf32>>
-    fir.freemem %52 : !fir.heap<!fir.array<?xf32>>
+    fir.freemem %48 : !fir.heap<!fir.array<?xf32>>
     fir.store %10 to %11 : !fir.ref<!fir.box<!fir.heap<!fir.array<?xf32>>>>
   }

Does anything change after that first run with the additional CSE'd loads in the if branches? No:

> rg "Number of operations [DCS]+E" mine2.mlir dev2.mlir
mine2.mlir
18542:  (S) 123 num-cse'd - Number of operations CSE'd
18543:  (S)  28 num-dce'd - Number of operations DCE'd
18557:  (S) 0 num-cse'd - Number of operations CSE'd
18558:  (S) 0 num-dce'd - Number of operations DCE'd
18564:  (S) 0 num-cse'd - Number of operations CSE'd
18565:  (S) 0 num-dce'd - Number of operations DCE'd
18587:  (S) 1 num-cse'd - Number of operations CSE'd
18588:  (S) 0 num-dce'd - Number of operations DCE'd

dev2.mlir
18716:  (S) 117 num-cse'd - Number of operations CSE'd
18717:  (S)  28 num-dce'd - Number of operations DCE'd
18731:  (S) 0 num-cse'd - Number of operations CSE'd
18732:  (S) 0 num-dce'd - Number of operations DCE'd
18738:  (S) 0 num-cse'd - Number of operations CSE'd
18739:  (S) 0 num-dce'd - Number of operations DCE'd
18761:  (S) 0 num-cse'd - Number of operations CSE'd
18762:  (S) 0 num-dce'd - Number of operations DCE'd