[nvptx] Use no,yes for attribute predicable

[nvptx] Use no,yes for attribute predicable

The documentation states about the predicable instruction attribute:
...
This attribute must be a boolean (i.e. have exactly two elements in its
list-of-values), with the possible values being no and yes.
...

The nvptx port has instead:
...
(define_attr "predicable" "false,true"
  (const_string "true"))
...

Fix this by updating to:
...
(define_attr "predicable" "no,yes"
  (const_string "yes"))
...

Tested on nvptx.

gcc/ChangeLog:

2022-03-08  Tom de Vries  <tdevries@suse.de>

	PR target/104840
	* config/nvptx/nvptx.md (define_attr "predicable"): Use no,yes instead
	of false,true.