cmd/compile/internal/gc: remove isforw predicate table (cleanup)

Was only ever filled with one Etype (TFORW) and only used
in one place. Easier to just check for TFORW.

Change-Id: Icc96da3a22b0af1d7e60bc5841c744916c53341e
Reviewed-on: https://go-review.googlesource.com/c/147285


Reviewed-by: Martin Möhrmann <moehrmann@google.com>