Add a much more conservative strategy for aligning branch targets.

Previously, MBP essentially aligned every branch target it could. This
bloats code quite a bit, especially non-looping code which has no real
reason to prefer aligned branch targets so heavily.

As Andy said in review, it's still a bit odd to do this without a real
cost model, but this at least has much more plausible heuristics.

Fixes PR13265.

git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@161409 91177308-0d34-0410-b5e6-96231b3b80d8

test/Transforms/LoopStrengthReduce/X86/2012-01-13-phielim.ll

@@ -99,7 +99,6 @@ while.end:                                        ; preds = %entry
 ; CHECK: %for.body3.lr.ph.us.i.loopexit
 ; CHECK-NEXT: in Loop: Header
 ; CHECK-NEXT: incq
-; CHECK-NEXT: .align
 ; CHECK-NEXT: %for.body3.us.i
 ; CHECK-NEXT: Inner Loop
 ; CHECK: testb