middle-end/106331 - fix mem attributes for string op arguments
get_memory_rtx tries hard to come up with a MEM_EXPR to record
in the memory attributes but in the last fallback fails to properly
account for an unknown offset and thus, as visible in this testcase,
incorrect alignment computed from set_mem_attributes. The following
rectifies both parts.
PR middle-end/106331
* builtins.cc (get_memory_rtx): Compute alignment from
the original address and set MEM_OFFSET to unknown when
we create a MEM_EXPR from the base object of the address.