X86: support double extension of f16 type.

x86 has no native ability to extend an f16 to f64, but the same result
is obtained if we expand it into two separate extensions: f16 -> f32
-> f64.

Unfortunately the same is not true for truncate, so that still results
in a compilation failure.

git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@213251 91177308-0d34-0410-b5e6-96231b3b80d8
CodeGen: generate single libcall for fptrunc -> f16 operations.

Previously we asserted on this code. Currently compiler-rt doesn't
actually implement any of these new libcalls, but external help is
pretty much the only viable option for LLVM.

I've followed the much more generic "__truncST2" naming, as opposed to
the odd name for f32 -> f16 truncation. This can obviously be changed
later, or overridden by any targets that need to.

git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@213252 91177308-0d34-0410-b5e6-96231b3b80d8
[TABLEGEN] Do not crash on intrinsics with names longer than 40 characters

Differential Revision: http://reviews.llvm.org/D4537

git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@213253 91177308-0d34-0410-b5e6-96231b3b80d8
ARM: support direct f16 <-> f64 conversions

ARMv8 has instructions to handle it, otherwise a libcall is needed.

git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@213254 91177308-0d34-0410-b5e6-96231b3b80d8