gdb/infrun: move a 'regcache_read_pc' call down to first use

gdb/infrun: move a 'regcache_read_pc' call down to first use

In infrun.c's resume_1 function, move the definition of the local
variable PC down to its first use.  This is useful if the thread we want
to resume is already gone with a pending exit event, because we avoid
the error we would see otherwise when trying to read the PC.

gdb/ChangeLog:
2020-05-14  Tankut Baris Aktemur  <tankut.baris.aktemur@intel.com>

	* infrun.c (resume_1): Move a 'regcache_read_pc' call down to first
	use.