Darwin : Fix out-of-bounds access to df_regs_ever_live.

Darwin : Fix out-of-bounds access to df_regs_ever_live.

During changes made for LRA (or, perhaps, even before) we omitted
a check that the current register we are working on is a hard reg
before we tried to note its liveness.

A stage 1 built with fsanitize=address catches this, as does any
attempt to build master with clang and -std=c++11.

gcc/ChangeLog:

	* config/darwin.c (machopic_legitimize_pic_address): Check
	that the current pic register is one of the hard reg set
	before setting liveness.