Add CLOBBER_HIGH expression

Add CLOBBER_HIGH expression

Includes documentation.

2018-08-06  Alan Hayward  <alan.hayward@arm.com>

	* doc/rtl.texi (clobber_high): Add.
	(parallel): Add in clobber high
	* rtl.c (rtl_check_failed_code3): Add function.
	* rtl.def (CLOBBER_HIGH): Add expression.
	* rtl.h (RTL_CHECKC3): Add macro.
	(rtl_check_failed_code3): Add declaration.
	(XC3EXP): Add macro.

From-SVN: r263326