RISC-V: zicond: Fix opt2 pattern

RISC-V: zicond: Fix opt2 pattern

Fixes: 1d5bc3285e8a ("[committed][RISC-V] Fix 20010221-1.c with zicond")
This was tripping up gcc.c-torture/execute/pr60003.c at -O1 since in
failing case, pattern semantics were not matching with asm czero.nez

We start with the following src code snippet:

      if (a == 0)
	return 0;
      else
	return x;
    }

which is equivalent to:  "x = (a != 0) ? x : a" where x is NOT 0.
                                                ^^^^^^^^^^^^^^^^

and matches define_insn "*czero.nez.<GPR:mode><X:mode>.opt2"

| (insn 41 20 38 3 (set (reg/v:DI 136 [ x ])
|        (if_then_else:DI (ne (reg/v:DI 134 [ a ])
|                (const_int 0 [0]))
|            (reg/v:DI 136 [ x ])
|            (reg/v:DI 134 [ a ]))) {*czero.nez.didi.opt2}

The corresponding asm pattern generates
    czero.nez x, x, a   ; %0, %2, %1

which implies
    "x = (a != 0) ? 0 : a"

clearly not what the pattern wants to do.

Essentially "(a != 0) ? x : a" cannot be expressed with CZERO.nez if X
is not guaranteed to be 0.

However this can be fixed with a small tweak

"x = (a != 0) ? x : a"

   is same as

"x = (a == 0) ? a : x"

and since middle operand is 0 when a == 0, it is equivalent to

"x = (a == 0) ? 0 : x"

which can be expressed with CZERO.eqz

before fix		after fix
-----------------	-----------------
li        a5,1	        li        a5,1
ld        a4,8(sp)	ld        a4,8(sp)
czero.nez a0,a4,a5 	czero.eqz a0,a4,a5

The issue only happens at -O1 as at higher optimization levels, the
whole conditional move gets optimized away.

This fixes 4 testsuite failues in a zicond build:

FAIL: gcc.c-torture/execute/pr60003.c   -O1  execution test
FAIL: gcc.dg/setjmp-3.c execution test
FAIL: gcc.dg/torture/stackalign/setjmp-3.c   -O1  execution test
FAIL: gcc.dg/torture/stackalign/setjmp-3.c   -O1 -fpic execution test

gcc/ChangeLog:
	* config/riscv/zicond.md: Fix op2 pattern.

Signed-off-by: Vineet Gupta <vineetg@rivosinc.com>