Fix typo in gimple_fold_partial_load_store_mem_ref

Fix typo in gimple_fold_partial_load_store_mem_ref

As Robin spotted, my recent commit r13-3716 caused an ICE
on s390 if vector access with length is enabled there (his
patch for the enablement hasn't been committed yet).  The
failure is caused by one stupid typo, the bias on s390 is
-1, so the assertion should use tree_fits_shwi_p rather
than tree_fits_uhwi_p.  Thanks for Robin's catching.

I just reproduced the ICE and verified the fix worked fine
with a cross build, the optimized dump against the test case
gcc.target/powerpc/pr107412.c looked expected.

gcc/ChangeLog:

	* gimple-fold.cc (gimple_fold_partial_load_store_mem_ref): Use
	tree_fits_shwi_p rather than tree_fits_uhwi_p as bias is signed.