Don't combine fp_round (fp_round x) if f80 to f16 is generated

Summary:
This patch skips DAG combine of fp_round (fp_round x) if it results in
an fp_round from f80 to f16.

fp_round from f80 to f16 always generates an expensive (and as yet,
unimplemented) libcall to __truncxfhf2.  This prevents selection of
native f16 conversion instructions from f32 or f64.  Moreover, the first
(value-preserving) fp_round from f80 to either f32 or f64 may become a
NOP in platforms like x86.

Reviewers: ab

Subscribers: srhines, llvm-commits

Differential Revision: http://reviews.llvm.org/D17221

llvm-svn: 260769