[Hexagon] Remove dead constant assignment in hardware loop pass

After converting a loop to a hardware loop, the pass should remove
any unnecessary instructions from the old compare-and-branch
code. This patch removes a dead constant assignment that was
used in the compare instruction.

Differential Revision: http://reviews.llvm.org/D9720


git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@237373 91177308-0d34-0410-b5e6-96231b3b80d8

test/CodeGen/Hexagon/hwloop-ph-deadcode.ll

@@ -0,0 +1,23 @@
+; RUN: llc -march=hexagon -mcpu=hexagonv5 -O2 -disable-block-placement=0 < %s | FileCheck %s
+
+; Test that there is no redundant register assignment in the hardware loop
+; preheader.
+
+; CHECK-NOT: r{{.*}} = #5
+
+@g = external global i32
+
+define void @foo() #0 {
+entry:
+  br i1 undef, label %if.end38, label %for.body
+
+for.body:
+  %loopIdx.051 = phi i32 [ %inc, %for.body ], [ 0, %entry ]
+  store i32 1, i32* @g, align 4
+  %inc = add i32 %loopIdx.051, 1
+  %cmp9 = icmp ult i32 %inc, 5
+  br i1 %cmp9, label %for.body, label %if.end38
+
+if.end38:
+  ret void
+}