[RS6000] ICE in decompose, at rtl.h:2282

[RS6000] ICE in decompose, at rtl.h:2282

during RTL pass: fwprop1
gcc.dg/pr82596.c: In function 'test_cststring':
gcc.dg/pr82596.c:27:1: internal compiler error: in decompose, at rtl.h:2282

-m32 gcc/testsuite/gcc.dg/pr82596.c fails along with other tests after
applying rtx_cost patches, which exposed a backend bug.
legitimize_address when presented with the following address
	(plus (reg) (const_int 0x7ffffffff))
attempts to rewrite it as a high/low sum.  The low part is 0xffff, or
-1, making the high part 0x80000000.  But this is no longer canonical
for SImode.

	* config/rs6000/rs6000.c (rs6000_legitimize_address): Use
	gen_int_mode for high part of address constant.