This patch adds a macro that extracts the TYPE_MODE and forcibly
converts it to a scalar_float_mode.  The forcible conversion
includes a gcc_checking_assert that the mode is a SCALAR_FLOAT_MODE_P.

This becomes important as more static type checking is added by
later patches.  It has the additional benefit of bypassing the
VECTOR_TYPE_P (...) ? vector_type_mode (...) : ... condition
in TYPE_MODE; in release builds the new macro is a simple
field access.

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

gcc/
	* tree.h (SCALAR_FLOAT_TYPE_MODE): New macro.
	* builtins.c (expand_builtin_signbit): Use it instead of TYPE_MODE.
	* fold-const.c (fold_convert_const_real_from_fixed): Likewise.
	(native_encode_real): Likewise.
	(native_interpret_real): Likewise.
	* hsa-brig.c (emit_immediate_scalar_to_buffer): Likewise.
	* tree-vrp.c (simplify_float_conversion_using_ranges): Likewise.

gcc/cp/
	* mangle.c (write_real_cst): Use SCALAR_FLOAT_TYPE_MODE
	instead of TYPE_MODE.

gcc/fortran/
	* target-memory.c (size_float): Use SCALAR_FLOAT_TYPE_MODE
	instead of TYPE_MODE.

gcc/objc/
	* objc-encoding.c (encode_type): Use SCALAR_FLOAT_TYPE_MODE
	instead of TYPE_MODE.

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

From-SVN: r251460