Fix EXECUTE_IF_SET_IN_HARD_REG_SET use

Fix EXECUTE_IF_SET_IN_HARD_REG_SET use

As Maxim pointed out, r276309 broke -O3 bootstrap for aarch64-linux-gnu.
It turns out that EXECUTE_IF_SET_IN_HARD_REG_SET isn't safe to use for
temporary objects because their lifetimes don't extend to the end of
the loop.  This patch therefore uses an explicit temporary instead.

Other patches in the series already used this style (but for
full_reg_and_partial_clobbers rather than full_reg_clobbers),
so AFAICT this is the only place that needs fixing.

2019-10-06  Richard Sandiford  <richard.sandiford@arm.com>

gcc/
	* var-tracking.c (dataflow_set_clear_at_call): Hoist temporary
	function result outside of EXECUTE_IF_SET_IN_HARD_REG_SET.

From-SVN: r276639