Make isOperationLegal do what its name suggests, and introduce a

new isOperationLegalOrCustom, which does what isOperationLegal
previously did.

Update a bunch of callers to use isOperationLegalOrCustom
instead of isOperationLegal. In some case it wasn't obvious
which behavior is desired; when in doubt I changed then to
isOperationLegalOrCustom as that preserves their previous
behavior.

This is for the second half of PR3376.

llvm-svn: 63212