AMDGPU/GFX10: Don't generate s_code_end padding in the asm-printer

Summary:
The purpose of the padding is to guard against stale code being
fetched into the instruction cache by the lowest level prefetching.
We're generating relocatable ELF here, and so the padding should
arguably be added by the linker. This is in fact what Mesa does.

This also fixes multi-part shaders for Mesa.

Change-Id: I6bfede58f20e9f337762ccf39ef9e0e263e69e82

Reviewers: arsenm, rampitec, t-tye

Subscribers: kzhuravl, jvesely, wdng, yaxunl, dstuttard, tpr, llvm-commits

Tags: #llvm

Differential Revision: https://reviews.llvm.org/D63427

llvm-svn: 363602