[X86] Teach how to custom lower double-to-half conversions under fast-math.

[X86] Teach how to custom lower double-to-half conversions under fast-math.

This patch teaches the backend how to expand a double-half conversion into
a double-float conversion immediately followed by a float-half conversion.
We do this only under fast-math, and if float-half conversions are legal
for the target.

Added test CodeGen/X86/fastmath-float-half-conversion.ll

Differential Revision: http://reviews.llvm.org/D7832

git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@230276 91177308-0d34-0410-b5e6-96231b3b80d8