This patch enables generation of fused multiply add/sub for instructions operating on fp16.
Tested on aarch64-linux.
There are 7 CHECK-FIXME for patterns for which I was not able to create a testcase to exercise the added code paths.
Those 7 patterns are mixing v[4|8]i16 with v[4|8]fp16 types with the help of a bitcast.
I am not sure how to write a testcase without the bitcast, and to generate coverage over those combine patterns,
so I would appreciate help on rewriting those testcases.