[llvm][LoongArch] Fix compiler warning produced by assert

Fixes #154918

tion ‘llvm::SDValue fillSubVectorFromBuildVector(llvm::BuildVectorSDNode*, llvm::SelectionDAG&, llvm::SDLoc, const llvm::LoongArchSubtarget&, llvm::EVT, unsigned int)’:
/home/davspi01/work/open_source/llvm-project/llvm/lib/Target/LoongArch/LoongArchISelLowering.cpp:2863:16: warning: comparison of unsigned expression in ‘>= 0’ is always true [-Wtype-limits]
 2863 |   assert(first >= 0 &&
      |          ~~~~~~^~~~

first is unsigned so this part of the expression is redundant.

Author: DavidSpickett

llvm/lib/Target/LoongArch/LoongArchISelLowering.cpp

@@ -2860,8 +2860,7 @@ static SDValue fillSubVectorFromBuildVector(BuildVectorSDNode *Node,
                                             EVT ResTy, unsigned first) {
   unsigned NumElts = ResTy.getVectorNumElements();
 
-  assert(first >= 0 &&
-         first + NumElts <= Node->getSimpleValueType(0).getVectorNumElements());
+  assert(first + NumElts <= Node->getSimpleValueType(0).getVectorNumElements());
 
   SmallVector<SDValue, 16> Ops(Node->op_begin() + first,
                                Node->op_begin() + first + NumElts);