SH_FILE_EXPANSION broken with process substitution

[Oliver Kiddle <okiddle@xxxxxxxxxxx>]

Following on from the gpg completion breakage, it seems this option has
been broken when used in combination with EQUALS.

This got broken in 35a8612 (workers/26042), released in 4.3.10:
  zsh -df --shfileexpansion -c ': =(echo hi)'

The _gpg issue is minimally demonstrated with the following. It is
reproducible even with 3.0.8 so may be by design.

  zsh -df --shfileexpansion -c 'a=( ${x%[:=\[]*} )'

The documentation only talks about tilde expansion not equals expansion
so it isn't obvious why this should fail. Or why swapping : and = fixes
it. This perhaps goes to show that the option isn't widely used other
than as part of emulate sh (which also disables equals).

Oliver