widen mult: Fix handling of _Fract mixed with _Fract [PR119568]

The problem here is we try calling find_widening_optab_handler_and_mode
with to_mode=E_USAmode and from_mode=E_UHQmode. This causes an ICE (with checking only).
The fix is to reject the case where the mode classes are different in convert_plusminus_to_widen
before even trying to deal with the modes.

Bootstrapped and tested on x86_64-linux-gnu.

	PR tree-optimization/119568

gcc/ChangeLog:

	* tree-ssa-math-opts.cc (convert_plusminus_to_widen): Reject different
	mode classes.

Signed-off-by: Andrew Pinski <andrew.pinski@oss.qualcomm.com>