[SDAG] Harden assumption in getMemsetStringVal

In 5235973, an ICE was fixed in
getMemsetStringVal where f128 wasn't handled. It was noted at the time
[1] that the code below this also looks suspect, since it assumes the
element type of VT is either an f32 or f64.

This part of getMemsetStringVal relates to memcpy operations where the
source is a copy from a zero constant. The VT in question is determined
by TargetLowering::findOptimalMemOpLowering, which in turn calls a
further TLI hook getOptimalMemOpType.

For AArch64, getOptimalMemOpType returns either a v16i8, f128, i64, i32
or Other. For Other, TargetLowering::findOptimalMemOpLowering will then
pick an integer VT. So on AArch64 at least, I don't believe the suspect
code can be reached.

For other targets, ARM and x86 are the only ones that return a FP vector
type from getOptimalMemOpType. For both targets, the only such type is
v2f64, but given f64 is already handled it should also be fine.

To defend this, I considered adding an assert as mentioned in [1], but
given getConstantFP handles vector types, I figured using this to fully
handle the FP types makes the code simpler and more robust.

For test coverage I added unreachables to both of the branches handling
FP types in this code, but found neither fired with check-llvm across
all targets.

Test coverage was added to llvm/test/CodeGen/AArch64/memcpy-f128.ll in
5235973 to defend ICE on f128, but at
some point it stopped hitting this code.

AArch64TargetLowering::getOptimalMemOpType was updated in
2920061, so I suspect this is when it
happened, although I haven't verified this. Although I did find by
updating the test to disable NEON, getOptimalMemOpType returns an f128
and the branch is once again hit.

For the final branch noted as suspect in [1], as far as I can tell this
has never had any test coverage, so I've added a test to the ARM backend
for this.

Fixes: #20521 [1]

Author: c-rhodes

llvm/lib/CodeGen/SelectionDAG/SelectionDAG.cpp

@@ -8015,16 +8015,8 @@ static SDValue getMemsetStringVal(EVT VT, const SDLoc &dl, SelectionDAG &DAG,
   if (Slice.Array == nullptr) {
     if (VT.isInteger())
       return DAG.getConstant(0, dl, VT);
-    if (VT == MVT::f32 || VT == MVT::f64 || VT == MVT::f128)
+    if (VT.isFloatingPoint())
       return DAG.getConstantFP(0.0, dl, VT);
-    if (VT.isVector()) {
-      unsigned NumElts = VT.getVectorNumElements();
-      MVT EltVT = (VT.getVectorElementType() == MVT::f32) ? MVT::i32 : MVT::i64;
-      return DAG.getNode(ISD::BITCAST, dl, VT,
-                         DAG.getConstant(0, dl,
-                                         EVT::getVectorVT(*DAG.getContext(),
-                                                          EltVT, NumElts)));
-    }
     llvm_unreachable("Expected type!");
   }