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

Darwin, rs6000: 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.

We now split the Darwin high/low selectors into two:
 1. One that handles non-PIC addresses (kernel mode, mdynamic-no-pic).
 2. One that handles PIC addresses and rejects SYMBOL_REFs unless they are
    suitably wrapped in the MACHOPIC_OFFSET unspec.

The second case is handled by providing a new predicate (macho_pic_address)
that checks the requirements.

Backported from 4c3792d448964f7bd99e7eac2c29c9eb7c2bfb84 and
f1b3e3853329b58fb2e50c17487df2ecbc4a5608

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.c (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.
	(rs6000_legitimize_address): Do not apply the TLS processing to
	Darwin.
	* config/rs6000/darwin.md (@machopic_high_<mode>): New.
	(@machopic_low_<mode>): New.
	* config/rs6000/predicates.md (macho_pic_address): New.