Fix test fails on powerpc-darwin.

The current implementation of “speculation_barrier”
and “group_end_nop” insns emit hard-wired register
names which causes tests using them to fail on Darwin,
at least, which uses “rNN” instead of “NN”.

The patch makes the register names for these insns use
the operand output mechanism to substitute the
appropriate variant when needed.

gcc/

2019-04-21  Iain Sandoe  <iain@sandoe.co.uk>

	* config/rs6000/rs6000.md (group_end_nop): Emit insn register
	names using operand format, rather than hard-wired.
	(speculation_barrier): Likewise.

From-SVN: r270480