powerpc: Handle DImode rotatert implemented with rlwinm (PR69946)

powerpc: Handle DImode rotatert implemented with rlwinm (PR69946)

Some DImode rotate-right-and-mask can be implemented best with a rlwinm
instruction: those that could be a lshiftrt instead of a rotatert, while
the mask is not right-aligned.  Why the rotate in the testcase is not
optimised to a plain shift is another question, but we need to handle
it here anyway.  We compute the shift amount for a 64-bit rotate.  This
is 32 too high in this case; if we print using %h that is masked out (and
this doesn't silently let through invalid instructions, everything is
checked by rs6000_is_valid_shift_mask which is much more thorough).

	PR target/69946
	* config/rs6000/rs6000.c (rs6000_insn_for_shift_mask): Print rlwinm
	shift amount using %h.  Add comment.

gcc/testsuite/
	* gcc.target/powerpc/pr69946.c: New file.

From-SVN: r233755