Checker: Allow don't-care output on a line

This patch changes the behaviour of whitespace characters in CHECK
lines, allowing for additional content between verified parts of the
matched output line. Tests therefore won't need to explicitly match
attributes which are not tested.

The way attributes are printed ensures that the right part of the
line is matched against.

Example:
  - output line:   i32 Div [ i4 i8 ] ( loop_header:null )
  - CHECK before:  Div [ {{i\d+}} {{i\d+}} ] ( loop_header:null )
  - CHECK now:     Div ( loop_header:null )

Change-Id: Icf6bacfb285ae288bea21640e860a871a94cc386