LOOP-UNROLL: Leverage HAS_SIGNED_ZERO for var expansion

The insert_var_expansion_initialization depends on the
HONOR_SIGNED_ZEROS to initialize the unrolling variables
to +0.0f when -0.0f and no-signed-option.  Unfortunately,
we should always keep the -0.0f here because:

* The -0.0f is always the correct initial value.
* We need to support the target that always honor signed zero.

Thus, we need to leverage MODE_HAS_SIGNED_ZEROS when initialize
instead of HONOR_SIGNED_ZEROS.  Then the target/backend can
decide to honor the no-signed-zero or not.

We also removed the testcase pr30957-1.c, as it makes undefined behavior
whether the return value is positive or negative.

The below tests are passed for this patch:

* The riscv regression tests.
* The aarch64 regression tests.
* The x86 bootstrap and regression tests.

gcc/ChangeLog:

	* loop-unroll.cc (insert_var_expansion_initialization): Leverage
	MODE_HAS_SIGNED_ZEROS for expansion variable initialization.

gcc/testsuite/ChangeLog:

	* gcc.dg/pr30957-1.c: Remove.

Signed-off-by: Pan Li <pan2.li@intel.com>