Gray out branch-likely delay slots that match branch target-1

[simonlindholm]

if both LHS and RHS match the pattern

branchlikely .target
  x

x
.target:

it's worth not highlighting diffs for the first x, because it's just an artifact of an assembler optimization; the real diff is at the second x. We used to do this unconditionally, because IDO only ever emits branch likelies of this form, but with GCC it needs actual pattern matching.