LRA, rs6000, Darwin: Amend lo_sum use for forced constants [PR104117].

Two issues resulted in this PR, which manifests when we force a constant into
memory in LRA (in PIC code on Darwin).  The presence of such forced constants
is quite dependent on other RTL optimisations, and it is easy for the issue to
become latent for a specific case.

First, in the Darwin-specific rs6000 backend code, we were not being careful
enough in rejecting invalid symbolic addresses.  Specifically, when generating
PIC code, we require a SYMBOL_REF to be wrapped in an UNSPEC_MACHOPIC_OFFSET.

Second, LRA was attempting to load a register using an invalid lo_sum address.

Signed-off-by: Iain Sandoe <iain@sandoe.co.uk>
Co-authored-by: Vladimir Makarov <vmakarov@redhat.com>

	PR target/104117

gcc/ChangeLog:

	* config/rs6000/rs6000.cc (darwin_rs6000_legitimate_lo_sum_const_p):
	Check for UNSPEC_MACHOPIC_OFFSET wrappers on symbolic addresses when
	emitting PIC code.
	(legitimate_lo_sum_address_p): Likewise.
	* lra-constraints.cc (process_address_1): Do not attempt to emit a reg
	load from an invalid lo_sum address.