LoongArch: Fix infinite secondary reloading of FCCmode [PR113148]

The GCC internal doc says:

     X might be a pseudo-register or a 'subreg' of a pseudo-register,
     which could either be in a hard register or in memory.  Use
     'true_regnum' to find out; it will return -1 if the pseudo is in
     memory and the hard register number if it is in a register.

So "MEM_P (x)" is not enough for checking if we are reloading from/to
the memory.  This bug has caused reload pass to stall and finally ICE
complaining with "maximum number of generated reload insns per insn
achieved", since r14-6814.

Check if "true_regnum (x)" is -1 besides "MEM_P (x)" to fix the issue.

gcc/ChangeLog:

	PR target/113148
	* config/loongarch/loongarch.cc (loongarch_secondary_reload):
	Check if regno == -1 besides MEM_P (x) for reloading FCCmode
	from/to FPR to/from memory.

gcc/testsuite/ChangeLog:

	PR target/113148
	* gcc.target/loongarch/pr113148.c: New test.

gcc/config/loongarch/loongarch.cc

@@ -6906,7 +6906,8 @@ loongarch_secondary_reload (bool in_p ATTRIBUTE_UNUSED, rtx x,
 	  return NO_REGS;
 	}
 
-      if (reg_class_subset_p (rclass, FP_REGS) && MEM_P (x))
+      if (reg_class_subset_p (rclass, FP_REGS)
+	  && (regno == -1 || MEM_P (x)))
 	return GR_REGS;
 
       return NO_REGS;

gcc/testsuite/gcc.target/loongarch/pr113148.c

@@ -0,0 +1,44 @@
+/* PR 113148: ICE caused by infinite reloading */
+/* { dg-do compile } */
+/* { dg-options "-O2 -march=la464 -mfpu=64 -mabi=lp64d" } */
+
+struct bound
+{
+  double max;
+} drawQuadrant_bound;
+double w4, innerXfromXY_y, computeBound_right_0;
+struct arc_def
+{
+  double w, h;
+  double a0, a1;
+};
+static void drawQuadrant (struct arc_def *);
+static void
+computeBound (struct arc_def *def, struct bound *bound)
+{
+  double ellipsex_1, ellipsex_0;
+  bound->max = def->a1 ?: __builtin_sin (w4) * def->h;
+  if (def->a0 == 5 && def->w == def->h)
+    ;
+  else
+    ellipsex_0 = def->a0 == 0.0 ?: __builtin_cos (w4);
+  if (def->a1 == 5 && def->w == def->h)
+    ellipsex_1 = bound->max;
+  __builtin_sqrt (ellipsex_1 * innerXfromXY_y * innerXfromXY_y * w4);
+  computeBound_right_0 = ellipsex_0;
+}
+void
+drawArc ()
+{
+  struct arc_def foo;
+  for (;;)
+    drawQuadrant (&foo);
+}
+void
+drawQuadrant (struct arc_def *def)
+{
+  int y, miny;
+  computeBound (def, &drawQuadrant_bound);
+  while (y >= miny)
+    ;
+}