ifcvt: Fix ICE with (fix:SI (fix:DF (reg:DF))) [PR117712]

As documented in the manual, FIX/UNSIGNED_FIX from floating point
mode to integral mode has unspecified rounding and FIX from floating point
mode to the same floating point mode is expressing rounding toward zero.
So, some targets (arc, arm, csky, m68k, mmix, nds32, pdp11, sparc and
visium) use
(fix:SI (fix:SF (match_operand:SF 1 "..._operand")))
etc. to express the rounding toward zero during conversion to integer.
For some reason other targets don't use that.

Anyway, the 2 FIXes (or inner FIX with outer UNSIGNED_FIX) cause problems
since the r15-2890 which removed some strict checks in ifcvt.cc on what
SET_SRC can be actually conditionalized (I must say I'm still worried
about the change, don't know why one can't get e.g. inline asm or
something with UNSPEC or some complex backend specific RTLs that
force_operand can't handle), force_operand just ICEs on it, it can only
handle (through expand_fix) conversions from floating point to integral.

The following patch fixes this by detecting this case and just pretend
the inner FIX isn't there, i.e. call expand_fix with the inner FIX's
operand instead, which works and on targets like arm it will just
create the nested FIXes again.

2025-02-28  Jakub Jelinek  <jakub@redhat.com>

	PR rtl-optimization/117712
	* expr.cc (force_operand): Handle {,UNSIGNED_}FIX with
	FIX operand using expand_fix on the inner FIX operand.

	* gcc.dg/pr117712.c: New test.