Update LLVM to llvm/llvm-project@ac8bb735

Update LLVM to llvm/llvm-project@ac8bb735. C++ changes are related to
change in behavior of TypeConverter. It used to generate
UnrealizedConversionCastOp, during applySignatureConversion in
GenericOpTypePropagation of TypePropagationPass.cpp, however now it's
not. This causes unrealized_conversion_cast to be generated later and
hence survive the pass. To repro above behavior, try undo the C++ change
in this PR and then:

```
wget https://gist.githubusercontent.com/raikonenfnu/dfb3b274007df8c4be87daf9ee67a5f4/raw/e48cc07e5fa558cd2c450b0e3ae46568136e1be6/type_propagate_repro.mlir
iree-opt --pass-pipeline='builtin.module(func.func(iree-codegen-type-propagation))' propagate_test.mlir -o /dev/null

error: failed to legalize unresolved materialization from ('i8') to ('i1') that remained live after conversion
  ^bb0(%in: i1, %in_0: f32, %in_1: f32, %out: f32):
       ^
propagate_test.mlir:5:8: note: see current operation: %10 = "builtin.unrealized_conversion_cast"(%arg0) : (i8) -> i1
propagate_test.mlir:6:11: note: see existing live user here: %10 = arith.select %9, %in_0, %in_1 : f32
```

This PR also carries the following reverts:

llvm/llvm-project#120999
llvm/llvm-project#120115
llvm/llvm-project#119461

The main issue with this PR(12099 and 120115) is it breaks matvec codegen generating scf.if instead of scf.for(s). An issue will be pushed up for repro.

The main issue with PR 119461 is it breaks e2e riscv test by making it get stuck on infinite loop.
```
/path/to/iree-build/tools/iree-compile --output-format=vm-bytecode --mlir-print-op-on-diagnostic=false --iree-hal-target-backends=llvm-cpu --iree-input-type=stablehlo --iree-input-demote-f64-to-f32 --iree-llvmcpu-target-cpu=generic /path/to/iree/tests/e2e/stablehlo_ops/three_fry.mlir -o three_fly_exec_target.mlir --iree-llvmcpu-target-triple=riscv64 --iree-llvmcpu-target-abi=lp64d --iree-llvmcpu-target-cpu-features=+m,+a,+d,+zvl512b,+v --mlir-disable-threading
> infinite loop
```

Signed-off-by: Stanley Winata <stanley.winata@amd.com>

compiler/src/iree/compiler/Codegen/Common/TypePropagationPass.cpp

@@ -79,6 +79,13 @@ static std::optional<Type> getLegalizedType(Type t) {
 
 namespace {
 
+/// Materialize
+Value materializeAsConvertElementType(OpBuilder &builder, Type type,
+                                  ValueRange inputs, Location loc) {
+  assert(inputs.size() == 1 && "expected exactly one input");
+  return convertElementType(builder, loc, type, inputs[0]);
+}
+
 /// Type converter to use for type propagation.
 struct TypePropagationTypeConverter : public TypeConverter {
   TypePropagationTypeConverter() {
@@ -526,11 +533,9 @@ struct TypePropagationPass final
     RewritePatternSet patterns(context);
 
     TypePropagationTypeConverter typeConverter;
-    typeConverter.addArgumentMaterialization(
-        [&](OpBuilder &builder, Type type, ValueRange inputs, Location loc) {
-          assert(inputs.size() == 1 && "expected exactly one input");
-          return convertElementType(builder, loc, type, inputs[0]);
-        });
+    typeConverter.addArgumentMaterialization(materializeAsConvertElementType);
+    typeConverter.addSourceMaterialization(materializeAsConvertElementType);
+    typeConverter.addTargetMaterialization(materializeAsConvertElementType);
 
     patterns.insert<
         ConstantOpTypeConversion, ForwardSourceType<arith::ExtUIOp>,