rs6000: Fix stack clash for big residuals (PR85287)

rs6000: Fix stack clash for big residuals (PR85287)

The stack clash protection code had a logic error in how it decided
whether to put the final update size in a register, or to emit it
directly in an insn.  This fixes it.  It also tidies some surrounding
code.

	PR target/85287
	* gcc/config/rs6000/rs6000.md (allocate_stack): Put the residual size
	for stack clash protection in a register whenever we need it to be in
	a register.

From-SVN: r259299