Fix -ffast-math flags handling inconsistencies

Fix -ffast-math flags handling inconsistencies

This patch implements the following set of changes:

1. If a component flag of -ffast-math (or -funsafe-math-optimizations)
   is explicitly set (or reset) on the command line, this should override
   any implicit change due to -f(no-)fast-math, no matter in which order
   the flags come on the command line.  This change affects all flags.

2. Any component flag modified from its default by -ffast-math should
   be reset to the default by -fno-fast-math.  This was previously
   not done for the following flags:
      -fcx-limited-range
      -fexcess-precision=

3. Once -ffinite-math-only is true, the -f(no-)signaling-nans flag has
   no meaning (if we have no NaNs at all, it does not matter whether
   there is a difference between quiet and signaling NaNs).  Therefore,
   it does not make sense for -ffast-math to imply -fno-signaling-nans.
   (This is also a documentation change.)

4. -ffast-math is documented to imply -fno-rounding-math, however the
   latter setting is the default anyway; therefore it does not make
   sense to try to modify it from its default setting.

5. The __FAST_MATH__ preprocessor macro should be defined if and only
   if all the component flags of -ffast-math are set to the value that
   is documented as the effect of -ffast-math.  The following flags
   were currently *not* so tested:
     -fcx-limited-range
     -fassociative-math
     -freciprocal-math
     -frounding-math
   (Note that we should still *test* for -fno-rounding-math here even
   though it is not set as to 4.  -ffast-math -frounding-math should
   not set the __FAST_MATH__ macro.)
   This is also a documentation change.

2020-11-24  Ulrich Weigand  <uweigand@de.ibm.com>

gcc/

	* doc/invoke.texi (-ffast-math): Remove mention of -fno-signaling-nans.
	Clarify conditions when __FAST_MATH__ preprocessor macro is defined.

	* opts.c (common_handle_option): Pass OPTS_SET to set_fast_math_flags
	and set_unsafe_math_optimizations_flags.
	(set_fast_math_flags): Add OPTS_SET argument, and use it to avoid
	setting flags already explicitly set on the command line.  In the !set
	case, also reset x_flag_cx_limited_range and x_flag_excess_precision.
	Never reset x_flag_signaling_nans or x_flag_rounding_math.
	(set_unsafe_math_optimizations_flags): Add OPTS_SET argument, and use
	it to avoid setting flags already explicitly set on the command line.
	(fast_math_flags_set_p): Also test x_flag_cx_limited_range,
	x_flag_associative_math, x_flag_reciprocal_math, and
	x_flag_rounding_math.