RTL_SSA: Relax PHI_MODE in phi_setup

Hi, Richard.

RISC-V port needs to add a bunch VLS modes (V16QI,V32QI,V64QI,...etc)
There are sharing same REG_CLASS with VLA modes (VNx16QI,VNx32QI,...etc)

When I am adding those VLS modes, the RTL_SSA initialization in VSETVL PASS (inserted after RA) ICE:
rvv.c:13:1: internal compiler error: in partial_subreg_p, at rtl.h:3186
   13 | }
      | ^
0xf7a5b1 partial_subreg_p(machine_mode, machine_mode)
        ../../../riscv-gcc/gcc/rtl.h:3186
0x1407616 wider_subreg_mode(machine_mode, machine_mode)
        ../../../riscv-gcc/gcc/rtl.h:3252
0x2a2c6ff rtl_ssa::combine_modes(machine_mode, machine_mode)
        ../../../riscv-gcc/gcc/rtl-ssa/internals.inl:677
0x2a2b9a4 rtl_ssa::function_info::simplify_phi_setup(rtl_ssa::phi_info*, rtl_ssa::set_info**, bitmap_head*)
        ../../../riscv-gcc/gcc/rtl-ssa/functions.cc:146
0x2a2c142 rtl_ssa::function_info::simplify_phis()
        ../../../riscv-gcc/gcc/rtl-ssa/functions.cc:258
0x2a2b3f0 rtl_ssa::function_info::function_info(function*)
        ../../../riscv-gcc/gcc/rtl-ssa/functions.cc:51
0x1cebab9 pass_vsetvl::init()
        ../../../riscv-gcc/gcc/config/riscv/riscv-vsetvl.cc:4578
0x1cec150 pass_vsetvl::execute(function*)
        ../../../riscv-gcc/gcc/config/riscv/riscv-vsetvl.cc:4716

The reason is that we have V32QImode (size = [32,0]) which is the mode set as regno_reg_rtx[97]
When the PHI input def comes from ENTRY BLOCK (index =0), the def->mode () = V32QImode.
But the phi_mode = VNx2QI for example (I use VLA modes intrinsic write the codes).
Then combine_modes report ICE.

gcc/ChangeLog:

	* rtl-ssa/internals.inl: Fix when mode1 and mode2 are not ordred.

gcc/rtl-ssa/internals.inl

@@ -673,6 +673,9 @@ combine_modes (machine_mode mode1, machine_mode mode2)
   if (mode2 == E_BLKmode)
     return mode1;
 
+  if (!ordered_p (GET_MODE_SIZE (mode1), GET_MODE_SIZE (mode2)))
+    return BLKmode;
+
   return wider_subreg_mode (mode1, mode2);
 }