[flang][NFC] turn (h)fir.declare side effect into debug ressource alloca (llvm#113321)

See https://reviews.llvm.org/D157626 for the rational of declare having
side effects.

The write effect is to scary for passes that look for read/write effects
without caring about the resource affected. I know Slava asked for it,
but I think the creation of the `DebuggingResource` was enough and that
a write is too much. The alloca effect is sufficient to prevent DCE to
remove it, which is all we care about currently.

This currently is flag as a reason for creating LHS temporary in
assignment to vector subscripted entity with array constructor.
There is a lot of read/write side effect analysis in the
"lower-hlfir-ordered-assignments" pass, and I feel like we will just
keep adding weird "debug ressource" bypassing here and there with these
side effects.

Author: jeanPerier

flang/include/flang/Optimizer/Dialect/FIROps.td

@@ -3078,7 +3078,7 @@ def fir_IsPresentOp : fir_SimpleOp<"is_present", [NoMemoryEffect]> {
 // debug information so we would like to keep this around even if the value
 // is not used.
 def fir_DeclareOp : fir_Op<"declare", [AttrSizedOperandSegments,
-    MemoryEffects<[MemWrite<DebuggingResource>]>,
+    MemoryEffects<[MemAlloc<DebuggingResource>]>,
     DeclareOpInterfaceMethods<fir_FortranVariableOpInterface>]> {
   let summary = "declare a variable";
 

flang/include/flang/Optimizer/HLFIR/HLFIROps.td

@@ -36,7 +36,7 @@ class hlfir_Op<string mnemonic, list<Trait> traits>
 // from the declare operation can be used to generate debug information so we
 // don't want to remove it as dead code
 def hlfir_DeclareOp : hlfir_Op<"declare", [AttrSizedOperandSegments,
-    MemoryEffects<[MemWrite<DebuggingResource>]>,
+    MemoryEffects<[MemAlloc<DebuggingResource>]>,
     DeclareOpInterfaceMethods<fir_FortranVariableOpInterface>]> {
   let summary = "declare a variable and produce an SSA value that can be used as a variable in HLFIR operations";