RISC-V: Disable AVL propagation of vrgather instruction

RISC-V: Disable AVL propagation of vrgather instruction

This patch fixes following FAILs in zvl1024b of both RV32/RV64:

FAIL: gcc.c-torture/execute/990128-1.c   -O2  execution test
FAIL: gcc.c-torture/execute/990128-1.c   -O2 -flto -fno-use-linker-plugin -flto-partition=none  execution test
FAIL: gcc.c-torture/execute/990128-1.c   -O2 -flto -fuse-linker-plugin -fno-fat-lto-objects  execution test
FAIL: gcc.c-torture/execute/990128-1.c   -O3 -fomit-frame-pointer -funroll-loops -fpeel-loops -ftracer -finline-functions  execution test
FAIL: gcc.c-torture/execute/990128-1.c   -O3 -g  execution test
FAIL: gcc.dg/torture/pr58955-2.c   -O3 -fomit-frame-pointer -funroll-loops -fpeel-loops -ftracer -finline-functions  execution test

The root case can be simpliy described in this following small case:

https://godbolt.org/z/7GaxbEGzG

typedef int64_t v1024b __attribute__ ((vector_size (128)));

void foo (void *out, void *in, int64_t a, int64_t b)
{
  v1024b v = {a,a,a,a,a,a,a,a,a,a,a,a,a,a,a,a};
  v1024b v2 = {b,b,b,b,b,b,b,b,b,b,b,b,b,b,b,b};
  v1024b index = *(v1024b*)in;
  v1024b v3 = __builtin_shuffle (v, v2, index);
  __riscv_vse64_v_i64m1 (out, (vint64m1_t)v3, 10);
}

Incorrect ASM:

foo:
        li      a5,31
        vsetivli        zero,10,e64,m1,ta,mu
        vmv.v.x v2,a5
        vl1re64.v       v1,0(a1)
        vmv.v.x v4,a2
        vand.vv v1,v1,v2
        vmv.v.x v3,a3
        vmsgeu.vi       v0,v1,16
        vrgather.vv     v2,v4,v1       --> AVL = VLMAX according to codes.
        vadd.vi v1,v1,-16
        vrgather.vv     v2,v3,v1,v0.t  --> AVL = VLMAX according to codes.
        vse64.v v2,0(a0)               --> AVL = 10 according to codes.
        ret

For vrgather dest, source, index instruction, when index may has the value > the following store AVL
that is index value > 10.  In this situation, the codes above will end up with:

The source vector of vrgather has undefined value on index >= AVL (which is 10 in this case).

So disable AVL propagation for vrgather instruction.

	PR target/112599
	PR target/112670

gcc/ChangeLog:

	* config/riscv/riscv-avlprop.cc (alv_can_be_propagated_p): New function.
	(vlmax_ta_p): Disable vrgather AVL propagation.

gcc/testsuite/ChangeLog:

	* gcc.target/riscv/rvv/autovec/pr112599-1.c: New test.