AMDGPU: Merge S_BUFFER_LOAD_DWORD_IMM into x2, x4

AMDGPU: Merge S_BUFFER_LOAD_DWORD_IMM into x2, x4

Summary:
Only constant offsets (*_IMM opcodes) are merged.
It reuses code for LDS load/store merging.
It relies on the scheduler to group loads.

The results are mixed, I think they are mostly positive. Most shaders are
affected, so here are total stats only:

 SGPRS: 2072198 -> 2151462 (3.83 %)
 VGPRS: 1628024 -> 1634612 (0.40 %)
 Spilled SGPRs: 7883 -> 8942 (13.43 %)
 Spilled VGPRs: 97 -> 101 (4.12 %)
 Scratch size: 1488 -> 1492 (0.27 %) dwords per thread
 Code Size: 60222620 -> 52940672 (-12.09 %) bytes
 Max Waves: 374337 -> 373066 (-0.34 %)

There is 13.4% increase in SGPR spilling, DiRT Showdown spills a few more
VGPRs (now 37), but 12% decrease in code size.

These are the new stats for SGPR spilling. We already spill a lot SGPRs,
so it's uncertain whether more spilling will make any difference since
SGPRs are always spilled to VGPRs:

 SGPR SPILLING APPS   Shaders SpillSGPR AvgPerSh
 alien_isolation         2938       100      0.0
 batman_arkham_origins    589         6      0.0
 bioshock-infinite       1769         4      0.0
 borderlands2            3968        22      0.0
 counter_strike_glob..   1142        60      0.1
 deus_ex_mankind_div..   1410        79      0.1
 dirt-showdown            533         4      0.0
 dirt_rally               364      1163      3.2
 divinity                1052         2      0.0
 dota2                   1747         7      0.0
 f1-2015                  776      1515      2.0
 grid_autosport          1767      1505      0.9
 hitman                  1413       273      0.2
 left_4_dead_2           1762         4      0.0
 life_is_strange         1296        26      0.0
 mad_max                  358        96      0.3
 metro_2033_redux        2670        60      0.0
 payday2                 1362        22      0.0
 portal                   474         3      0.0
 saints_row_iv           1704         8      0.0
 serious_sam_3_bfe        392      1348      3.4
 shadow_of_mordor        1418        12      0.0
 shadow_warrior          3956       239      0.1
 talos_principle          324      1735      5.4
 thea                     172        17      0.1
 tomb_raider             1449       215      0.1
 total_war_warhammer      242        56      0.2
 ue4_effects_cave         295        55      0.2
 ue4_elemental            572        12      0.0
 unigine_tropics          210        56      0.3
 unigine_valley           278       152      0.5
 victor_vran             1262        84      0.1
 yofrankie                 82         2      0.0

Reviewers: arsenm, nhaehnle

Subscribers: kzhuravl, wdng, yaxunl, dstuttard, tpr, llvm-commits, t-tye

Differential Revision: https://reviews.llvm.org/D38949

llvm-svn: 317751