R600/SI: Fix bug from insertion of llvm.SI.end.cf into loop headers

R600/SI: Fix bug from insertion of llvm.SI.end.cf into loop headers

The llvm.SI.end.cf intrinsic is used to mark the end of if-then blocks,
if-then-else blocks, and loops.  It is responsible for updating the
exec mask to re-enable threads that had been masked during the preceding
control flow block.  For example:

s_mov_b64 exec, 0x3                 ; Initial exec mask
s_mov_b64 s[0:1], exec              ; Saved exec mask
v_cmpx_gt_u32 exec, s[2:3], v0, 0   ; llvm.SI.if
do_stuff()
s_or_b64 exec, exec, s[0:1]         ; llvm.SI.end.cf

The bug fixed by this patch was one where the llvm.SI.end.cf intrinsic
was being inserted into the header of loops.  This would happen when
an if block terminated in a loop header and we would end up with
code like this:

s_mov_b64 exec, 0x3                 ; Initial exec mask
s_mov_b64 s[0:1], exec              ; Saved exec mask
v_cmpx_gt_u32 exec, s[2:3], v0, 0   ; llvm.SI.if
do_stuff()

LOOP:                       ; Start of loop header
s_or_b64 exec, exec, s[0:1] ; llvm.SI.end.cf <-BUG: The exec mask has the
                              same value at the beginning of each loop
			      iteration.
do_stuff();
s_cbranch_execnz LOOP

The fix is to create a new basic block before the loop and insert the
llvm.SI.end.cf there.  This way the exec mask is restored before the
start of the loop instead of at the beginning of each iteration.

git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@228302 91177308-0d34-0410-b5e6-96231b3b80d8