Fix isNegatibleForFree to not return true for ConstantFP nodes

after legalize.  Just because a constant is legal (e.g. 0.0 in SSE) 
doesn't mean that its negated value is legal (-0.0).  We could make
this stronger by checking to see if the negated constant is actually
legal post negation, but it doesn't seem like a big deal.


git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@47591 91177308-0d34-0410-b5e6-96231b3b80d8

test/CodeGen/Generic/2008-02-25-NegateZero.ll

@@ -0,0 +1,14 @@
+; RUN: llvm-as < %s | llc 
+; rdar://5763967
+
+define void @test() {
+entry:
+	%tmp98 = load float* null, align 4		; <float> [#uses=1]
+	%tmp106 = load float* null, align 4		; <float> [#uses=1]
+	%tmp113 = add float %tmp98, %tmp106		; <float> [#uses=1]
+	%tmp119 = sub float %tmp113, 0.000000e+00		; <float> [#uses=1]
+	call void (i32, ...)* @foo( i32 0, float 0.000000e+00, float %tmp119 ) nounwind 
+	ret void
+}
+
+declare void @foo(i32, ...)