rtlgen: Always copy register:value to temp in continuation.

rtlgen: Always copy register:value to temp in continuation.

This is necessary because register:value may be a machine register
available for allocation as a machine temporary (as on x86 where it
is rax).

I had found most (if not all) other cases where register:value needed
to be stashed in a pseudo temporary in a continuation, but I missed
this one: if the continuation is a predicate continuation that first
restores a dynamic link, the code to restore the dynamic link would
ask for a temporary register that might (and did) turn out to be the
same as register:value.  Example:

(define (rexists pred thing)
  (let tlp ((thing thing))
    (pp thing)
    (cond ((pred thing) #t)
          ((vector? thing)
           (pp 'here)
           (let ((n (vector-length thing)))
	     (let lp ((i 0))
	       (cond ((fix:= i n) #f)
		     ((tlp (vector-ref thing i))        ; (*)
                      (pp 'there)
                      #t)
		     (else (lp (fix:+ i 1)))))))
          ((list? thing)
           (any tlp thing))
          (else #f))))

The predicate continuation marked (*) had the following code
generated for it (rax is (r 0), where return value lives on entry):

continuation-11:
	;; (assign (register #x2d) (post-increment (register 4) 1))
	(pop q (r 0))
	;; (assign (register #x2e) (object->address (register #x2d)))
	(and q (r 0) (r 5))
	;; (assign (offset (register 6) (machine-constant 4)) (register #x2e))
	(mov q (@ro 6 #x20) (r 0))
	;; (eq-test (register 0) (constant #f))
	(cmp q (r 0) (&u 0))
	(jne (@pcr label-42))

This clobbers rax to restore the dynamic link, so the comparison at
the end compares compares the dynamic link, not the return value as
intended, to #f.

(cherry picked from commit f38c4ff4c048955585416a774e61352fcd9e5178)