PR debug/55717
	* rtlhooks-def.h (RTL_HOOKS_GEN_LOWPART_NO_EMIT): Define to
	gen_lowpart_if_possible.
	(gen_lowpart_no_emit_general): Remove prototype.
	* rtlhooks.c (gen_lowpart_no_emit_general): Removed.
	* simplify-rtx.c (simplify_unary_operation_1, 
	simplify_binary_operation_1): Continue simplifying if
	rtl_hooks.gen_lowpart_no_emit returns NULL_RTX.
	* dwarf2out.c (mem_loc_descriptor) <case TRUNCATE>: Handle
	truncation like lowpart SUBREG.

	* testsuite/g++.dg/opt/pr55717.C: New test.

From-SVN: r194575