[PR114991][IRA]: Improve reg equiv invariant calculation

[PR114991][IRA]: Improve reg equiv invariant calculation

In PR test case IRA preferred to allocate hard reg to a pseudo instead
of its equivalence.  This resulted in allocating caller-saved hard reg
and generating save/restore insns in the function prologue/epilogue.
The equivalence is an invariant (stack pointer plus offset) and the
pseudo is used mostly as memory address.  This happened as there was
no simplification of insn after the invariant substitution.  The patch
adds the necessary code.

gcc/ChangeLog:

	PR target/114991
	* ira-costs.cc (equiv_can_be_consumed_p): Add new argument invariant_p.
	Add code for dealing with the invariant.
	(calculate_equiv_gains): Don't consider init insns.  Pass the new
	argument to equiv_can_be_consumed_p.  Don't treat invariant as
	memory.

gcc/testsuite/ChangeLog:

	PR target/114991
	* gcc.target/aarch64/pr114991.c: New test.