[LLDB] On AArch64, reconfigure register context first (llvm#70742)

On an SVE/SME the register context configuration may change after the
inferior process has executed. This was handled via
https://reviews.llvm.org/D159504 but it is reconfiguring and clearing
the register context after we've parsed any expedited reigster values
from the stop reply packet. That results in lldb having to read each
register value one at a time while at that stop location, which will be
a performance problem on non-local debug setups.

The configuration & clearing needs to happen first. Also, update the
names of the local variables for a little clarity.

(cherry picked from commit d402645)

Author: jasonmolenda

lldb/source/Plugins/Process/gdb-remote/ProcessGDBRemote.cpp

@@ -1642,9 +1642,20 @@ ThreadSP ProcessGDBRemote::SetThreadStopInfo(
   }
 
   ThreadGDBRemote *gdb_thread = static_cast<ThreadGDBRemote *>(thread_sp.get());
-  RegisterContextSP gdb_reg_ctx_sp(gdb_thread->GetRegisterContext());
+  RegisterContextSP reg_ctx_sp(gdb_thread->GetRegisterContext());
 
-  gdb_reg_ctx_sp->InvalidateIfNeeded(true);
+  reg_ctx_sp->InvalidateIfNeeded(true);
+
+  // AArch64 SVE/SME specific code below updates SVE and ZA register sizes and
+  // offsets if value of VG or SVG registers has changed since last stop.
+  const ArchSpec &arch = GetTarget().GetArchitecture();
+  if (arch.IsValid() && arch.GetTriple().isAArch64()) {
+    GDBRemoteRegisterContext *gdb_remote_reg_ctx =
+        static_cast<GDBRemoteRegisterContext *>(reg_ctx_sp.get());
+
+    if (gdb_remote_reg_ctx)
+      gdb_remote_reg_ctx->AArch64SVEReconfigure();
+  }
 
   auto iter = std::find(m_thread_ids.begin(), m_thread_ids.end(), tid);
   if (iter != m_thread_ids.end())
@@ -1655,24 +1666,10 @@ ThreadSP ProcessGDBRemote::SetThreadStopInfo(
     WritableDataBufferSP buffer_sp(
         new DataBufferHeap(reg_value_extractor.GetStringRef().size() / 2, 0));
     reg_value_extractor.GetHexBytes(buffer_sp->GetData(), '\xcc');
-    uint32_t lldb_regnum = gdb_reg_ctx_sp->ConvertRegisterKindToRegisterNumber(
+    uint32_t lldb_regnum = reg_ctx_sp->ConvertRegisterKindToRegisterNumber(
         eRegisterKindProcessPlugin, pair.first);
     gdb_thread->PrivateSetRegisterValue(lldb_regnum, buffer_sp->GetData());
   }
-
-  // AArch64 SVE specific code below calls AArch64SVEReconfigure to update
-  // SVE register sizes and offsets if value of VG register has changed
-  // since last stop.
-  const ArchSpec &arch = GetTarget().GetArchitecture();
-  if (arch.IsValid() && arch.GetTriple().isAArch64()) {
-    GDBRemoteRegisterContext *reg_ctx_sp =
-        static_cast<GDBRemoteRegisterContext *>(
-            gdb_thread->GetRegisterContext().get());
-
-    if (reg_ctx_sp)
-      reg_ctx_sp->AArch64SVEReconfigure();
-  }
-
   thread_sp->SetName(thread_name.empty() ? nullptr : thread_name.c_str());
 
   gdb_thread->SetThreadDispatchQAddr(thread_dispatch_qaddr);