Remove debug metadata from copied instruction to prevent GPUModule verification failure

Summary:
**Remove debug metadata from instruction to be copied to prevent the source file's debug metadata being copied into GPUModule and eventually failing Module verification and ASM string codegeneration.**

When copying the instruction onto the Module meant for the GPU, debug metadata attached to an instruction causes all related metadata to be pulled into the Module, including the DICompileUnit, which is not listed in llvm.dbg.cu of the Module. This fails the verification of the Module and generation of the ASM string.

The only debug metadata of the instruction, the DebugLoc, is unset by this patch.

This patch reattempts https://reviews.llvm.org/D35630 by targeting only those instructions that are to end up in a Module meant for the GPU.

Reviewers: grosser, bollu

Reviewed By: grosser

Subscribers: pollydev

Tags: #polly

Differential Revision: https://reviews.llvm.org/D36161

llvm-svn: 309822