MIPS: Use signaling fcmp instructions for LT/LE/LTGT

MIPS: Use signaling fcmp instructions for LT/LE/LTGT

LT/LE: c.lt.fmt/c.le.fmt on pre-R6 and cmp.lt.fmt/cmp.le.fmt have
different semantic:
   c.lt.fmt will signal for all NaN, including qNaN;
   cmp.lt.fmt will only signal sNaN, while not qNaN;
   cmp.slt.fmt has the same semantic as c.lt.fmt;
   lt/le of RTL will signaling qNaN.

while in `s<code>_<SCALARF:mode>_using_<FPCC:mode>`, RTL operation
`lt`/`le` are convert to c/cmp's lt/le, which is correct for C.cond.fmt,
while not for CMP.cond.fmt. Let's convert them to slt/sle if ISA_HAS_CCF.

For LTGT, which signals qNaN, `sne` of r6 has same semantic, while pre-R6
has only inverse one `ngl`.  Thus for RTL we have to use the `uneq` as the
operator, and introduce a new CC mode: CCEmode to mark it as signaling.

This patch can fix
   gcc.dg/torture/pr91323.c for pre-R6;
   gcc.dg/torture/builtin-iseqsig-* for R6.

gcc:
	* config/mips/mips-modes.def: New CC_MODE CCE.
	* config/mips/mips-protos.h(mips_output_compare): New function.
	* config/mips/mips.cc(mips_allocate_fcc): Set CCEmode count=1.
	(mips_emit_compare): Use CCEmode for LTGT/LT/LE for pre-R6.
	(mips_output_compare): New function. Convert lt/le to slt/sle
	for R6; convert ueq to ngl for CCEmode.
	(mips_hard_regno_mode_ok_uncached): Mention CCEmode.
	* config/mips/mips.h: Mention CCEmode for LOAD_EXTEND_OP.
	* config/mips/mips.md(FPCC): Add CCE.
	(define_mode_iterator MOVECC): Mention CCE.
	(define_mode_attr reg): Add CCE with "z".
	(define_mode_attr fpcmp): Add CCE with "c".
	(define_code_attr fcond): ltgt should use sne instead of ne.
	(s<code>_<SCALARF:mode>_using_<FPCC:mode>): call mips_output_compare.