i386: Return CCmode from ix86_cc_mode for unknown RTX code [PR112494]

Combine wants to combine following instructions into an insn that can
perform both an (arithmetic) operation and set the condition code.  During
the conversion a new RTX is created, and combine passes the RTX code of the
innermost RTX expression of the CC use insn in which CC reg is used to
SELECT_CC_MODE, to determine the new mode of the comparison:

Trying 5 -> 8:
    5: r98:DI=0xd7
    8: flags:CCZ=cmp(r98:DI,0)
      REG_EQUAL cmp(0xd7,0)
Failed to match this instruction:
(parallel [
        (set (reg:CC 17 flags)
            (compare:CC (const_int 215 [0xd7])
                (const_int 0 [0])))
        (set (reg/v:DI 98 [ flags ])
            (const_int 215 [0xd7]))
    ])

where:

(insn 5 2 6 2 (set (reg/v:DI 98 [ flags ])
        (const_int 215 [0xd7])) "pr112494.c":8:8 84 {*movdi_internal}
     (nil))

(insn 8 7 11 2 (set (reg:CCZ 17 flags)
        (compare:CCZ (reg/v:DI 98 [ flags ])
            (const_int 0 [0]))) "pr112494.c":11:9 8 {*cmpdi_ccno_1}
     (expr_list:REG_EQUAL (compare:CCZ (const_int 215 [0xd7])
            (const_int 0 [0]))
        (nil)))

x86_cc_mode (AKA SELECT_CC_MODE) is not prepared to handle random RTX
codes and triggers gcc_unreachable() when SET RTX code is passed to it.
The patch removes gcc_unreachable() and returns CCmode for unknown
RTX codes, so combine can try various combinations involving CC reg
without triggering ICE.

Please note that x86 MOV instructions do not set flags, so the above
combination is not recognized as a valid x86 instruction.

	PR target/112494

gcc/ChangeLog:

	* config/i386/i386.cc (ix86_cc_mode) [default]: Return CCmode.

gcc/testsuite/ChangeLog:

	* gcc.target/i386/pr112494.c: New test.