[PR118663][LRA]: Change secondary memory mode only if there are regs holding the...

[PR118663][LRA]: Change secondary memory mode only if there are regs holding the changed mode

  My recent patch for PR118067 changes the secondary memory mode if
all regs of the pseudo reg class are prohibited in the secondary mode.
But the patch does not check a special case when the
corresponding target hook returns this mode although there are no hard
regs of pseudo class holding value of the mode at all.  This results
in given PR and this patch fixes it.

gcc/ChangeLog:

	PR target/118663
	* lra-constraints.cc (invalid_mode_reg_p): Check empty
	reg_class_contents.

gcc/testsuite/ChangeLog:

	PR target/118663
	* gcc.target/powerpc/pr118663.c: New.