MMIX: Handle LRA FP-to-SP-elimination oddity

MMIX: Handle LRA FP-to-SP-elimination oddity

When LRA is in progress, it can try and validate insns
half-way through frame-pointer (FP) to stack-pointer (SP)
elimination.  Operands have then been substituted where the
offset is from the SP elimination but the register is the
(hard) frame-pointer:

lra-eliminations.cc:lra_eliminate_regs_1:370:
 rtx to = subst_p ? ep->to_rtx : ep->from_rtx;

In this regard reload played nicely.  Unfortunately, the
frame_pointer_operand predicate in mmix/predicates.md barfs
on such an address.  This broke the use of the MMIX
frame_pointer_operand predicate (and the Yf constraint),
used only in the nonlocal_goto_receiver expansion (which is
used in e.g. code generated for C++ "catch").

Force MMIX frame_pointer_operand to accept an FP+offset for
the duration of lra_in_progress.

	* config/mmix/predicates.md (frame_pointer_operand): Handle FP+offset
	when lra_in_progress.