[CodeGen] Don't pretend we can expand f16 libcalls.

[CodeGen] Don't pretend we can expand f16 libcalls.

We used to mark a bunch of libm nodes as Expand for f16.  There are no
libcalls we can use for those, so we eventually just hit an unhelpful
llvm_unreachable in ExpandFPLibCall.

Instead, just ignore them altogether.  If nothing else changes, we'll
then get the more descriptive and pleasant "Cannot select" fatal error.

There's an argument to be made for consistency, but f16 is already
special in all the good ways, and as long as there's no f16 support in
the ops expander (this patch), as well as the Soften/Expand float
legalizers (which, when hit, will currently segfault), I think there's
no point in even pretending we can legalize any of this.

This shouldn't affect anything that's not already broken.

git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@233328 91177308-0d34-0410-b5e6-96231b3b80d8