gimple-fold: Refine gimple_fold_partial_load_store_mem_ref [PR107412]

gimple-fold: Refine gimple_fold_partial_load_store_mem_ref [PR107412]

Following Richard's review comments, this patch is to use
untruncated type for the length used for IFN_LEN_{LOAD,STORE}
instead of "unsigned int" for better robustness.  It also
avoid to use to_constant and tree arithmetic for subtraction.

Co-authored-by: Richard Sandiford  <richard.sandiford@arm.com>
	PR tree-optimization/107412

gcc/ChangeLog:

	* gimple-fold.cc (gimple_fold_partial_load_store_mem_ref): Use
	untruncated type for the length, and avoid to_constant and tree
	arithmetic for subtraction.