R600/SI: Adjsut SGPR live ranges before register allocation

SGPRs are written by instructions that sometimes will ignore control flow,
which means if you have code like:

if (VGPR0) {
  SGPR0 = S_MOV_B32 0
} else {
  SGPR0 = S_MOV_B32 1
}

The value of SGPR0 will 1 no matter what the condition is.

In order to deal with this situation correctly, we need to view the
program as if it were a single basic block when we calculate the
live ranges for the SGPRs.  They way we actually update the live
range is by iterating over all of the segments in each LiveRange
object and setting the end of each segment equal to the start of
the next segment.  So a live range like:

[3888r,9312r:0)[10032B,10384B:0)  0@3888r

will become:

[3888r,10032B:0)[10032B,10384B:0)  0@3888r

This change will allow us to use SALU instructions within branches.

llvm-svn: 212215