[Hexagon] Use getSignedConstant for RoundTo8 XForm

To handle negative offset addrmodes correctly.

Fixes #117558 (comment).

Author: nikic

llvm/lib/Target/Hexagon/HexagonPatterns.td

@@ -2675,7 +2675,7 @@ def s30_2ProperPred  : PatLeaf<(i32 imm), [{
 }]>;
 def RoundTo8 : SDNodeXForm<imm, [{
   int32_t Imm = N->getSExtValue();
-  return CurDAG->getTargetConstant(Imm & -8, SDLoc(N), MVT::i32);
+  return CurDAG->getSignedTargetConstant(Imm & -8, SDLoc(N), MVT::i32);
 }]>;
 
 let AddedComplexity = 150 in

llvm/test/CodeGen/Hexagon/addrmode-neg-offset.ll

@@ -0,0 +1,9 @@
+; RUN: llc -mtriple=hexagon < %s | FileCheck %s
+
+; CHECK: r0 = add(r0,#4)
+; CHECK: memd(r0+#-16) =
+define void @neg_gep_store(ptr %ptr) {
+  %gep = getelementptr i8, ptr %ptr, i32 -12
+  store i64 0, ptr %gep, align 8
+  ret void
+}