tree-optimization: Fix use of uninitialized variables warnings [PR94952]

tree-optimization: Fix use of uninitialized variables warnings [PR94952]

While bootstrapping GCC on S/390 with --enable-checking=release several
warnings about use of uninitialized variables bitpos, bitregion_start, and
bitregion_end of function pass_store_merging::process_store are raised.
According to PR94952 these seem to be false positives which are silenced by
initialising the mentioned variables.

Bootstrapped on S/390.  Ok for master and releases/gcc-10 assuming that
regtest succeeds (still running but I don't see a reason why it
should fail)?

gcc/ChangeLog:

2020-05-18  Stefan Schulze Frielinghaus  <stefansf@linux.ibm.com>

	PR tree-optimization/94952
	* gimple-ssa-store-merging.c (pass_store_merging::process_store):
	Initialize variables bitpos, bitregion_start, and bitregion_end in
	order to silence warnings about use of uninitialized variables.