[AMDGPU] Assert previous SGPR exists when bundling preloaded args (llvm#118802)

This came up from a downstream static analysis tool.

Author: jchlanda

llvm/lib/Target/AMDGPU/SIISelLowering.cpp

@@ -2537,6 +2537,7 @@ void SITargetLowering::allocatePreloadKernArgSGPRs(
 
       // Arg is preloaded into the previous SGPR.
       if (ArgLoc.getLocVT().getStoreSize() < 4 && Alignment < 4) {
+        assert(InIdx >= 1 && "No previous SGPR");
         Info.getArgInfo().PreloadKernArgs[InIdx].Regs.push_back(
             Info.getArgInfo().PreloadKernArgs[InIdx - 1].Regs[0]);
         continue;