rs6000: cannot_force_const_mem for HIGH code rtx[PR106460]

As the issue in PR106460, a rtx 'high:DI (symbol_ref:DI ("var_48")' is tried
to store into constant pool and ICE occur.  But actually, this rtx represents
partial incomplete address and can not be put into a .rodata section.

This patch updates rs6000_cannot_force_const_mem to return true for rtx(s) with
HIGH code, because these rtx(s) indicate part of address and are not ok for
constant pool.

Below are some examples:
(high:DI (const:DI (plus:DI (symbol_ref:DI ("xx") (const_int 12 [0xc])))))
(high:DI (symbol_ref:DI ("var_1")..)))

	PR target/106460

gcc/ChangeLog:

	* config/rs6000/rs6000.cc (rs6000_cannot_force_const_mem): Return true
	for HIGH code rtx.

gcc/testsuite/ChangeLog:

	* gcc.target/powerpc/pr106460.c: New test.