r213378 improved f16 bitcasts, so that they go directly through subregs,
instead of through the stack. That code now causes an assertion failure
for bitcasts from other 16-bits types (most importantly v2i8).
Correct that by doing the custom lowering for i16 bitcasts only when the
input is an f16.
Part of PR21549.
Differential Revision: http://reviews.llvm.org/D6307
git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@223074 91177308-0d34-0410-b5e6-96231b3b80d8