RSX-Debugger: Implement backwards scrolling

* Use 2 points of known true RSX code roots and follow them in order to peek at the current section of valid RSX code:
These roots are: current RSX instruction address and the last targeted address by a branch instruction.

rpcs3/Emu/RSX/RSXThread.cpp

@@ -12,6 +12,7 @@
 #include "Capture/rsx_capture.h"
 #include "rsx_methods.h"
 #include "gcm_printing.h"
+#include "RSXDisAsm.h"
 #include "Emu/Cell/lv2/sys_event.h"
 #include "Emu/Cell/lv2/sys_time.h"
 #include "Emu/Cell/Modules/cellGcmSys.h"
@@ -2640,6 +2641,63 @@ namespace rsx
 		fifo_ctrl->sync_get();
 	}
 
+	std::pair<u32, u32> thread::try_get_pc_of_x_cmds_backwards(u32 count, u32 get) const
+	{
+		if (!ctrl)
+		{
+			return {0, umax};
+		}
+
+		if (!count)
+		{
+			return {0, get};
+		}
+
+		u32 true_get = ctrl->get;
+		u32 start = last_known_code_start;
+
+		RSXDisAsm disasm(cpu_disasm_mode::survey_cmd_size, vm::g_sudo_addr, 0, this);
+
+		std::vector<u32> pcs_of_valid_cmds;
+		pcs_of_valid_cmds.reserve(std::min<u32>((get - start) / 16, 0x4000)); // Rough estimation of final array size
+
+		auto probe_code_region = [&](u32 probe_start) -> std::pair<u32, u32>
+		{
+			pcs_of_valid_cmds.clear();
+			pcs_of_valid_cmds.push_back(probe_start);
+
+			while (pcs_of_valid_cmds.back() < get)
+			{
+				if (u32 advance = disasm.disasm(pcs_of_valid_cmds.back()))
+				{
+					pcs_of_valid_cmds.push_back(pcs_of_valid_cmds.back() + advance);
+				}
+				else
+				{
+					return {0, umax};
+				}
+			}
+
+			if (pcs_of_valid_cmds.size() == 1u || pcs_of_valid_cmds.back() != get)
+			{
+				return {0, umax};
+			}
+
+			u32 found_cmds_count = std::min(count, ::size32(pcs_of_valid_cmds) - 1);
+
+			return {found_cmds_count, *(pcs_of_valid_cmds.end() - 1 - found_cmds_count)};
+		};
+
+		auto pair = probe_code_region(start);
+
+		if (!pair.first)
+		{
+			pair = probe_code_region(true_get);
+		}
+
+		return pair;
+	}
+
 	void thread::recover_fifo(u32 line, u32 col, const char* file, const char* func)
 	{
 		const u64 current_time = rsx::uclock();