[AMDGPU] Promote constant offset to the immediate by finding a new base with 13bit...

[AMDGPU] Promote constant offset to the immediate by finding a new base with 13bit constant offset from the nearby instructions.

Summary: Promote constant offset to immediate by recomputing the relative 13bit offset from nearby instructions.
 E.g.
  s_movk_i32 s0, 0x1800
  v_add_co_u32_e32 v0, vcc, s0, v2
  v_addc_co_u32_e32 v1, vcc, 0, v6, vcc

  s_movk_i32 s0, 0x1000
  v_add_co_u32_e32 v5, vcc, s0, v2
  v_addc_co_u32_e32 v6, vcc, 0, v6, vcc
  global_load_dwordx2 v[5:6], v[5:6], off
  global_load_dwordx2 v[0:1], v[0:1], off
  =>
  s_movk_i32 s0, 0x1000
  v_add_co_u32_e32 v5, vcc, s0, v2
  v_addc_co_u32_e32 v6, vcc, 0, v6, vcc
  global_load_dwordx2 v[5:6], v[5:6], off
  global_load_dwordx2 v[0:1], v[5:6], off offset:2048

Author: FarhanaAleen

Reviewed By: arsenm, rampitec

Subscribers: llvm-commits, AMDGPU

Differential Revision: https://reviews.llvm.org/D55539

git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@349196 91177308-0d34-0410-b5e6-96231b3b80d8