AMDGPU: Fix instruction flags for s_endpgm

[arsenm]

Summary

Set isReturn, remove hasSideEffects. Also remove hasCtrlDep, I'm not really sure what that does.

[arsenm]

This requires disabling requiring structured CFG to fix a test regression from the post-RA tail duplication pass not running

[nhaehnle]

LGTM, given D23584.

[arsenm]

r285476

lib/Target/AMDGPU/SIInstructions.td

 def S_NOP : SOPP <0x00000000, (ins i16imm:$simm16), "s_nop $simm16">;
 
 let isTerminator = 1 in {
 
 def S_ENDPGM : SOPP <0x00000001, (ins), "s_endpgm",
   [(AMDGPUendpgm)]> {
   let simm16 = 0;
   let isBarrier = 1;
-  let hasCtrlDep = 1;
-  let hasSideEffects = 1;
+  let isReturn = 1;
 }
 
 let isBranch = 1 in {
 def S_BRANCH : SOPP <
   0x00000002, (ins sopp_brtarget:$simm16), "s_branch $simm16",
   [(br bb:$simm16)]> {
   let isBarrier = 1;
 }