[ira] avoid resetting ira_reg_equiv for function invariants

[ira] avoid resetting ira_reg_equiv for function invariants

An aarch64 toolchain built with --enable-default-pie fails
gcc.target/aarch64/sme/nonlocal_goto_[123].c because the register
allocator ends up resetting equivalences, so it concludes it needs to
preserve a rematerializable function invariant across a call instead
of rematerializing it.

Restore the find_reg_equiv_invariant_const logic that was in place
before commit 55a2c3226a3e90a6d65f19710bab1ac377054234 rewrote it and
added a (spurious?) requirement for non-function-invariants to retain
an equivalence when other PIC-related conditions didn't apply.  That
requirement seems either reversed or unnecessary.

for  gcc/ChangeLog

	* ira.cc (setup_reg_equiv): Retain function invariant
	equivalences.