GlobalISel: Move fconstant matching into tablegen

I don't really understand what the point of wip_match_opcode is.
It doesn't seem to have any purpose other than to list opcodes
to have all the logic in pure C++. You can't seem to  use it to
select multiple opcodes in the same way you use match.

Something is wrong with it, since the match emitter prints
"errors" if an opcode is covered by wip_match_opcode and
then appears in another pattern. For exmaple with this patch,
you see this several times in the build:

  error: Leaf constant_fold_fabs is unreachable
  note: Leaf idempotent_prop will have already matched

The combines are actually produced and the tests for them
do pass, so this seems to just be a broken warning.