[Transforms] Preserve metadata when converting invoke to call.

[Transforms] Preserve metadata when converting invoke to call.

The `changeToCall` function did not preserve the invoke's metadata.
Currently, there is probably no metadata that depends on being applied
on a CallInst or InvokeInst. Therefore we can replace the instruction's
metadata.

This fixes http://llvm.org/PR39994

Suggested-by: Moritz Kreutzer <moritz.kreutzer@siemens.com>
Differential Revision: https://reviews.llvm.org/D55666

git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@349170 91177308-0d34-0410-b5e6-96231b3b80d8