[legalizetypes] Push fp16 -> fp32 extension node to worklist.

Summary:
This way, the type legalization machinery will take care of registering
the result of this node properly.

This patches fixes all failing fp16 test cases  with expensive checks.
(CodeGen/ARM/fp16-promote.ll, CodeGen/ARM/fp16.ll, CodeGen/X86/cvt16.ll
CodeGen/X86/soft-fp.ll) 


Reviewers: t.p.northover, baldrick, olista01, bogner, jmolloy, davidxl, ab, echristo, hfinkel

Reviewed By: hfinkel

Subscribers: mehdi_amini, hfinkel, davide, RKSimon, aemerson, llvm-commits

Differential Revision: https://reviews.llvm.org/D28195

llvm-svn: 293765