Fix ALL_REGS thinko in initialisation of function_used_regs

Fix ALL_REGS thinko in initialisation of function_used_regs

My change to the -fipa-ra bookkeeping used ALL_REGS as the supposedly
safe default assumption, but ALL_REGS isn't literally all registers,
just a close approximation.

This caused a bootstrap failure on arm-linux-gnu, where the condition
code register isn't in ALL_REGS and so was being masked out of some
call-clobbered sets.

2019-10-02  Richard Sandiford  <richard.sandiford@arm.com>

gcc/
	* cgraph.c (cgraph_node::rtl_info): Use SET_HARD_REG_SET
	instead of reg_class_contents[ALL_REGS].

From-SVN: r276489