[MIPS] OperationAction for FP_TO_FP16, FP16_TO_FP

Summary:
Set operation action for FP16 conversion opcodes, so the Op legalizer
can choose the gnu_* libcalls for Mips.

Set LoadExtAction and TruncStoreAction for f16 scalars and vectors to
prevent (fpext (load )) and (store (fptrunc)) from getting combined into
unsupported operations.

Added test cases to test that these operations are handled correctly
for f16 scalars and vectors.  This patch depends on
http://reviews.llvm.org/D8755.

Reviewers: srhines

Subscribers: llvm-commits, ab

Differential Revision: http://reviews.llvm.org/D8804

llvm-svn: 235341