rtl-optimization/80960 - avoid creating garbage RTL in DSE

rtl-optimization/80960 - avoid creating garbage RTL in DSE

The following avoids repeatedly turning VALUE RTXen into
sth useful and re-applying a constant offset through get_addr
via DSE check_mem_read_rtx.  Instead perform this once for
all stores to be visited in check_mem_read_rtx.  This avoids
allocating 1.6GB of garbage PLUS RTXen on the PR80960
testcase, fixing the memory usage regression from old GCC.

2021-01-27  Richard Biener  <rguenther@suse.de>

	PR rtl-optimization/80960
	* dse.c (check_mem_read_rtx): Call get_addr on the
	offsetted address.