LoongArch: Implement C[LT]Z_DEFINED_VALUE_AT_ZERO

The LoongArch has defined ctz and clz on the backend, but if we want GCC
do CTZ transformation optimization in forwprop2 pass, GCC need to know
the value of c[lt]z at zero, which may be beneficial for some test cases
(like spec2017 deepsjeng_r).

After implementing the macro, we test dynamic instruction count on
deepsjeng_r:
- before 1688423249186
- after  1660311215745 (1.66% reduction)

gcc/ChangeLog:

	* config/loongarch/loongarch.h (CLZ_DEFINED_VALUE_AT_ZERO):
	Implement.
	(CTZ_DEFINED_VALUE_AT_ZERO): Same.

gcc/testsuite/ChangeLog:

	* gcc.dg/pr90838.c: add clz/ctz test support on LoongArch.

gcc/config/loongarch/loongarch.h

@@ -1239,3 +1239,8 @@ struct GTY (()) machine_function
 
 #define TARGET_EXPLICIT_RELOCS \
   (la_opt_explicit_relocs == EXPLICIT_RELOCS_ALWAYS)
+
+#define CLZ_DEFINED_VALUE_AT_ZERO(MODE, VALUE) \
+  ((VALUE) = GET_MODE_UNIT_BITSIZE (MODE), 2)
+#define CTZ_DEFINED_VALUE_AT_ZERO(MODE, VALUE) \
+  ((VALUE) = GET_MODE_UNIT_BITSIZE (MODE), 2)

gcc/testsuite/gcc.dg/pr90838.c

@@ -83,3 +83,8 @@ int ctz4 (unsigned long x)
 /* { dg-final { scan-assembler-times "ctz\t" 3 { target { rv32 } } } } */
 /* { dg-final { scan-assembler-times "andi\t" 1 { target { rv32 } } } } */
 /* { dg-final { scan-assembler-times "mul\t" 1 { target { rv32 } } } } */
+
+/* { dg-final { scan-tree-dump-times {= \.CTZ} 4 "forwprop2" { target { loongarch64*-*-* } } } } */
+/* { dg-final { scan-assembler-times "ctz.d\t" 1 { target { loongarch64*-*-* } } } } */
+/* { dg-final { scan-assembler-times "ctz.w\t" 3 { target { loongarch64*-*-* } } } } */
+/* { dg-final { scan-assembler-times "andi\t" 4 { target { loongarch64*-*-* } } } } */