Remove comments on CU skeleton construction, they're probably
obvious.

git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@198361 91177308-0d34-0410-b5e6-96231b3b80d8
[PPC] Fix comment to match function name

git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@198362 91177308-0d34-0410-b5e6-96231b3b80d8
[RegAlloc] Make tryInstructionSplit less aggressive.

The greedy register allocator tries to split a live-range around each
instruction where it is used or defined to relax the constraints on the entire
live-range (this is a last chance split before falling back to spill).
The goal is to have a big live-range that is unconstrained (i.e., that can use
the largest legal register class) and several small local live-range that carry
the constraints implied by each instruction.
E.g.,
Let csti be the constraints on operation i.

V1=
op1 V1(cst1)
op2 V1(cst2)

V1 live-range is constrained on the intersection of cst1 and cst2.

tryInstructionSplit relaxes those constraints by aggressively splitting each
def/use point:
V1=
V2 = V1
V3 = V2
op1 V3(cst1)
V4 = V2
op2 V4(cst2)

Because of how the coalescer infrastructure works, each new variable (V3, V4)
that is alive at the same time as V1 (or its copy, here V2) interfere with V1.
Thus, we end up with an uncoalescable copy for each split point.

To make tryInstructionSplit less aggressive, we check if the split point
actually relaxes the constraints on the whole live-range. If it does not, we do
not insert it.
Indeed, it will not help the global allocation problem:
- V1 will have the same constraints.
- V1 will have the same interference + possibly the newly added split variable
  VS.
- VS will produce an uncoalesceable copy if alive at the same time as V1.

<rdar://problem/15570057>


git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@198369 91177308-0d34-0410-b5e6-96231b3b80d8