middle-end: use inner variable when determining deferred FMA order [PR123898]

middle-end: use inner variable when determining deferred FMA order [PR123898]

If we defer an FMA creation the code tries to determine the order of the
operands before deferring.  To do this it compares the operands against the
result expression (which should contain the multiplication expression).

However the multiply might be wrapped in a conversion.  This change has us strip
one level of conversion (the most that convert_mult_to_fma) supports handling
and only then do the comparison.

We cannot strip ops[0] and ops[1] and store them stripped since after the
deferrence, if we create an FMA we need to know the original types and
convert_mult_to_fma handles the conversions during FMA creation anyway.

There's probably a similar helper to strip_nop_view_converts but I couldn't
find one, since many of the stripping helpers are recursive or don't support
stripping VIEW_CONVERTS.

gcc/ChangeLog:

	PR tree-optimization/123898
	* tree-ssa-math-opts.cc (strip_nop_view_converts): New.
	(convert_mult_to_fma): Use it.

gcc/testsuite/ChangeLog:

	PR tree-optimization/123898
	* gcc.target/aarch64/sve/pr123898.c: New test.