[amdgpu] Fix broken error detection in LDS lowering

[JonChesterfield]

Summary

std::optional<uint32_t> can be compared to uint32_t without warning, but does
not compare to the value within the optional. It needs to be prefixed *.
Wconversion does not warn about this.

bool bug(uint32_t Offset, std::optional<uint32_t> Expect)
{
  return (Offset != Expect);
}
bool deref(uint32_t Offset, std::optional<uint32_t> Expect)
{
  return (Offset != *Expect);
}

Both compile without warnings. Wrote the former, intended the latter.

llvm/lib/Target/AMDGPU/AMDGPUMachineFunction.cpp

if (isModuleEntryFunction()) {
     // other variables, e.g. dynamic lds, allocated after this call
 
     const GlobalVariable *GV = M->getNamedGlobal(ModuleLDSName);
     const GlobalVariable *KV = getKernelLDSGlobalFromFunction(F);
 
     if (GV && !canElideModuleLDS(F)) {
       unsigned Offset = allocateLDSGlobal(M->getDataLayout(), *GV, Align());
       std::optional<uint32_t> Expect = getLDSAbsoluteAddress(*GV);
-      if (!Expect || (Offset != Expect)) {
+      if (!Expect || (Offset != *Expect)) {
         report_fatal_error("Inconsistent metadata on module LDS variable");
       }
     }
 
     if (KV) {
       // The per-kernel offset is deterministic because it is allocated
       // before any other non-module LDS variables.
       unsigned Offset = allocateLDSGlobal(M->getDataLayout(), *KV, Align());
       std::optional<uint32_t> Expect = getLDSAbsoluteAddress(*KV);
-      if (!Expect || (Offset != Expect)) {
+      if (!Expect || (Offset != *Expect)) {
         report_fatal_error("Inconsistent metadata on kernel LDS variable");
       }
     }
   }
 }
 
 std::optional<uint32_t>
 AMDGPUMachineFunction::getLDSKernelIdMetadata(const Function &F) {