[LoopPeeling] Fix condition for phi-eliminating peeling

When peeling loops basing on phis becoming invariants, we make a wrong loop size check.
UP.Threshold should be compared against the total numbers of instructions after the transformation,
which is equal to 2 * LoopSize in case of peeling one iteration.
We should also check that the maximum allowed number of peeled iterations is not zero.

Reviewers: sanjoy, anna, reames, mkuper

Reviewed By: mkuper

Subscribers: llvm-commits

Differential Revision: https://reviews.llvm.org/D31753

llvm-svn: 300441
diff --git a/llvm/test/Transforms/LoopUnroll/peel-loop-not-forced.ll b/llvm/test/Transforms/LoopUnroll/peel-loop-not-forced.ll
index 3dcac87..c3cbbf1 100644
--- a/llvm/test/Transforms/LoopUnroll/peel-loop-not-forced.ll
+++ b/llvm/test/Transforms/LoopUnroll/peel-loop-not-forced.ll
@@ -1,4 +1,4 @@
-; RUN: opt < %s -S -loop-unroll -unroll-threshold=4 | FileCheck %s
+; RUN: opt < %s -S -loop-unroll -unroll-threshold=8 | FileCheck %s
 
 define i32 @invariant_backedge_1(i32 %a, i32 %b) {
 ; CHECK-LABEL: @invariant_backedge_1