[ARC] Use TARGET_CANNOT_SUBSTITUTE_MEM_EQUIV.

[ARC] Use TARGET_CANNOT_SUBSTITUTE_MEM_EQUIV.

Sometimes the memory equivalent is not valid due to a large offset.
For example replacing the ap register with its fp/sp-equivalent during
LRA step. To solve this we introduced TARGET_CANNOT_SUBSTITUTE_MEM_EQUIV.

gcc/
2017-08-08  Claudiu Zissulescu  <claziss@synopsys.com>

	* config/arc/arc.c (arc_cannot_substitute_mem_equiv_p): New function.
	(TARGET_CANNOT_SUBSTITUTE_MEM_EQUIV_P): Define.

gcc/testsuite
2017-08-08  Claudiu Zissulescu  <claziss@synopsys.com>

	* gcc.target/arc/lra-1.c: New test.

From-SVN: r255273