Remove unused variable to fix '[AMDGPU] modify named barrier builtins and intrinsics (#114550)'

#114550 caused a buildbot breakage (https://lab.llvm.org/buildbot/#/builders/66/builds/5853) because of an unused variable. This patch attempts to fix forward:

/home/b/sanitizer-x86_64-linux/build/llvm-project/llvm/lib/Target/AMDGPU/AMDGPUMachineFunction.cpp:106:24: error: variable 'TTy' set but not used [-Werror,-Wunused-but-set-variable]
106 |     if (TargetExtType *TTy = AMDGPU::isNamedBarrier(GV)) {
    |                        ^

Author: thurstond

llvm/lib/Target/AMDGPU/AMDGPUMachineFunction.cpp

@@ -103,7 +103,7 @@ unsigned AMDGPUMachineFunction::allocateLDSGlobal(const DataLayout &DL,
 
   unsigned Offset;
   if (GV.getAddressSpace() == AMDGPUAS::LOCAL_ADDRESS) {
-    if (TargetExtType *TTy = AMDGPU::isNamedBarrier(GV)) {
+    if (AMDGPU::isNamedBarrier(GV)) {
       std::optional<unsigned> BarAddr = getLDSAbsoluteAddress(GV);
       if (!BarAddr)
         llvm_unreachable("named barrier should have an assigned address");