rtl-ssa: Use right obstack for temporary allocation

rtl-ssa: Use right obstack for temporary allocation

I noticed while working on PR98863 that we were using the main
obstack to allocate temporary uses.  That was safe, but represents
a kind of local memory leak.

gcc/
	* rtl-ssa/accesses.cc (function_info::make_use_available): Use
	m_temp_obstack rather than m_obstack to allocate the temporary use.