Remove redundant variable from tree-call-cdce.c

Remove redundant variable from tree-call-cdce.c

shrink_wrap_one_built_in_call had both guard_bb and guard_bb0.
It looks like an earlier version of the pass may have updated
one of the variables in the while loop, but now they're just
two names for the same thing.

Tested on x86_64-linux-gnu, aarch64-linux-gnu and arm-linux-gnueabi.

gcc/
	* tree-call-cdce.c (shrink_wrap_one_built_in_call): Remove
	guard_bb0 and use guard_bb throughout.

From-SVN: r229532