[globalisel][tablegen] Enable the import of rules involving fma.

Summary:
G_FMA was recently added to GlobalISel which enables the import of rules
involving fma. Add the mapping to allow it.

Reviewers: ab, t.p.northover, qcolombet, rovka, aditya_nandakumar

Reviewed By: rovka

Subscribers: kristof.beyls, javed.absar, igorb, llvm-commits

Differential Revision: https://reviews.llvm.org/D35130

llvm-svn: 308308