[GlobalIsel] allow x86_fp80 values to be dumped.

Summary:
Otherwise the fallback path fails with an assertion on x86_64 targets,
when "x86_fp80" is encountered.

Reviewers: t.p.northover, zvi, guyblank

Reviewed By: zvi

Subscribers: rovka, kristof.beyls, llvm-commits

Differential Revision: https://reviews.llvm.org/D34975

llvm-svn: 307140