RISC-V: Make FRM as global register [PR118103]

RISC-V: Make FRM as global register [PR118103]

After we enabled the labe-combine pass after the mode-switching pass, it
will try to combine below insn patterns into op.  Aka:

(insn 40 5 41 2 (set (reg:SI 11 a1 [151])
  (reg:SI 69 frm)) "pr118103-simple.c":67:15 2712 {frrmsi}
  (nil))
(insn 41 40 7 2 (set (reg:SI 69 frm)
  (const_int 2 [0x2])) "pr118103-simple.c":69:8 2710 {fsrmsi_restore}
  (nil))
(insn 42 10 11 2 (set (reg:SI 69 frm)
  (reg:SI 11 a1 [151])) "pr118103-simple.c":70:8 2710 {fsrmsi_restore}
    (nil))

trying to combine definition of r11 in:
40: a1:SI=frm:SI
    into:
42: frm:SI=a1:SI
    instruction becomes a no-op:
(set (reg:SI 69 frm)
(reg:SI 69 frm))
original cost = 4 + 4 (weighted: 8.000000), replacement cost =
2147483647; keeping replacement
rescanning insn with uid = 42.
updating insn 42 in-place
verify found no changes in insn with uid = 42.
deleting insn 40

For example we have code as blow:
   9   │ int test_exampe () {
  10   │   test ();
  11   │
  12   │   size_t vl = 4;
  13   │   vfloat16m1_t va = __riscv_vle16_v_f16m1(a, vl);
  14   │   va = __riscv_vfnmadd_vv_f16m1_rm(va, va, va, __RISCV_FRM_RDN, vl);
  15   │   va = __riscv_vfmsac_vv_f16m1(va, va, va, vl);
  16   │
  17   │   __riscv_vse16_v_f16m1(b, va, vl);
  18   │
  19   │   return 0;
  20   │ }

it will be compiled to:
  53   │ main:
  54   │     addi    sp,sp,-16
  55   │     sd  ra,8(sp)
  56   │     call    initialize
  57   │     lui a6,%hi(b)
  58   │     lui a2,%hi(a)
  59   │     addi    a3,a6,%lo(b)
  60   │     addi    a2,a2,%lo(a)
  61   │     li  a4,4
  62   │ .L8:
  63   │     fsrmi   2
  64   │     vsetvli a5,a4,e16,m1,ta,ma
  65   │     vle16.v v1,0(a2)
  66   │     slli    a1,a5,1
  67   │     subw    a4,a4,a5
  68   │     add a2,a2,a1
  69   │     vfnmadd.vv  v1,v1,v1
  >> The fsrm a0 insn is deleted by late-combine <<
  70   │     vfmsub.vv   v1,v1,v1
  71   │     vse16.v v1,0(a3)
  72   │     add a3,a3,a1
  73   │     bgt a4,zero,.L8
  74   │     lh  a4,%lo(b)(a6)
  75   │     li  a5,-20480
  76   │     addi    a5,a5,-1382
  77   │     bne a4,a5,.L14
  78   │     ld  ra,8(sp)
  79   │     li  a0,0
  80   │     addi    sp,sp,16
  81   │     jr  ra

This patch would like to add the FRM register to the global_regs as it
is a cooperatively-managed global register.  And then the fsrm insn will
not be eliminated by late-combine.  The related spec17 cam4 failure may
also caused by this issue too.

The below test suites are passed for this patch.
* The rv64gcv fully regression test.

	PR target/118103

gcc/ChangeLog:

	* config/riscv/riscv.cc (riscv_conditional_register_usage): Add
	the FRM as the global_regs.

gcc/testsuite/ChangeLog:

	* gcc.target/riscv/rvv/base/pr118103-1.c: New test.
	* gcc.target/riscv/rvv/base/pr118103-run-1.c: New test.

Signed-off-by: Pan Li <pan2.li@intel.com>