Replace checks of:

  GET_MODE_CLASS (fmode) != MODE_FLOAT && !DECIMAL_FLOAT_MODE_P (fmode)

with !is_a <scalar_float_mode> and use MODE_CLASS equality/inequality
instead of:

  (GET_MODE_CLASS (tmode) == MODE_FLOAT && DECIMAL_FLOAT_MODE_P (fmode))
  || (GET_MODE_CLASS (fmode) == MODE_FLOAT && DECIMAL_FLOAT_MODE_P (tmode))

and:

  (GET_MODE_CLASS (tmode) == MODE_FLOAT
   && GET_MODE_CLASS (fmode) == MODE_FLOAT)
  || (DECIMAL_FLOAT_MODE_P (fmode) && DECIMAL_FLOAT_MODE_P (tmode))

2017-08-30  Richard Sandiford  <richard.sandiford@linaro.org>
	    Alan Hayward  <alan.hayward@arm.com>
	    David Sherwood  <david.sherwood@arm.com>

gcc/
	* optabs-libfuncs.c (gen_trunc_conv_libfunc): Use is_a
	<scalar_float_mode>.  Simplify.
	(gen_extend_conv_libfunc): Likewise.

Co-Authored-By: Alan Hayward <alan.hayward@arm.com>
Co-Authored-By: David Sherwood <david.sherwood@arm.com>

From-SVN: r251459