AMDGPU/SI: Turn off GPR Indexing Mode immediately after the interested instruction.

Summary:
  In the current implementation of GPR Indexing Mode when the index is of non-uniform, the s_set_gpr_idx_off instruction
is incorrectly inserted after the loop. This will lead the instructions with vgpr operands (v_readfirstlane for example) to read incorrect
vgpr.
 In this patch, we fix the issue by inserting s_set_gpr_idx_on/off immediately around the interested instruction.

Reviewers:
  rampitec

Differential Revision:
  https://reviews.llvm.org/D43297

llvm-svn: 325355