Split up subtracts into add+negate if they have a reassociable use or operand

that is also a subtract.  This implements PR2047 and Transforms/Reassociate/subtest2.ll 


git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@47241 91177308-0d34-0410-b5e6-96231b3b80d8

test/Transforms/Reassociate/subtest2.ll

@@ -0,0 +1,13 @@
+; With sub reassociation, constant folding can eliminate the uses of %a.
+;
+; RUN: llvm-as < %s | opt -reassociate -instcombine | llvm-dis | grep %a | count 1
+; PR2047
+
+define i32 @test(i32 %a, i32 %b, i32 %c) nounwind  {
+entry:
+	%tmp3 = sub i32 %a, %b		; <i32> [#uses=1]
+	%tmp5 = sub i32 %tmp3, %c		; <i32> [#uses=1]
+	%tmp7 = sub i32 %tmp5, %a		; <i32> [#uses=1]
+	ret i32 %tmp7
+}
+