rs6000: Fix ICE in decompose_normal_address. [PR93974]

Fix an ICE in decompose_normal_address(), which cannot handle Altivec AND:
addresses, by disallowing them via implementing the target hook
rs6000_cannot_substitute_mem_equiv_p.

gcc/
	PR rtl-optimization/93974
	* config/rs6000/rs6000.c (TARGET_CANNOT_SUBSTITUTE_MEM_EQUIV_P): Define.
	(rs6000_cannot_substitute_mem_equiv_p): New function.

gcc/testsuite/
	PR rtl-optimization/93974
	* g++.dg/pr93974.C: New test.