middle-end: Fix complex lowering of cabs with no LHS [PR120369]

middle-end: Fix complex lowering of cabs with no LHS [PR120369]

This was introduced by r15-1797-gd8fe4f05ef448e . I had missed that
the LHS of the cabs call could be NULL. This seems to only happen at -O0,
I tried to produce one that happens at -O1 but needed many different
options to prevent the removal of the call.
Anyways the fix is just keep around the call if the LHS is null.

Bootstrapped and tested on x86_64-linux-gnu.

	PR middle-end/120369

gcc/ChangeLog:

	* tree-complex.cc (gimple_expand_builtin_cabs): Return early
	if the LHS of cabs is null.

gcc/testsuite/ChangeLog:

	* gcc.dg/torture/pr120369-1.c: New test.

Signed-off-by: Andrew Pinski <quic_apinski@quicinc.com>
(cherry picked from commit 95c74f354ae3186e84fbada22d2e7f3845dbb659)