IPO: Add use-list-order verifier

Add a -verify-use-list-order pass, which shuffles use-list order, writes
to bitcode, reads back, and verifies that the (shuffled) order matches.

  - The utility functions live in lib/IR/UseListOrder.cpp.

  - Moved (and renamed) the command-line option to enable writing
    use-lists, so that this pass can return early if the use-list orders
    aren't being serialized.

It's not clear that this pass is the right direction long-term (perhaps
a separate tool instead?), but short-term it's a great way to test the
use-list order prototype.  I've added an XFAIL-ed testcase that I'm
hoping to get working pretty quickly.

This is part of PR5680.

git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@213945 91177308-0d34-0410-b5e6-96231b3b80d8

test/Bitcode/use-list-order.ll

@@ -0,0 +1,83 @@
+; RUN: opt -S < %s -preserve-bc-use-list-order -verify-use-list-order
+; XFAIL: *
+
+@a = global [4 x i1] [i1 0, i1 1, i1 0, i1 1]
+@b = alias i1* getelementptr ([4 x i1]* @a, i64 0, i64 2)
+
+define i64 @f(i64 %f) {
+entry:
+  %sum = add i64 %f, 0
+  ret i64 %sum
+}
+
+define i64 @g(i64 %g) {
+entry:
+  %sum = add i64 %g, 0
+  ret i64 %sum
+}
+
+define i64 @h(i64 %h) {
+entry:
+  %sum = add i64 %h, 0
+  ret i64 %sum
+}
+
+define i64 @i(i64 %i) {
+entry:
+  %sum = add i64 %i, 1
+  ret i64 %sum
+}
+
+define i64 @j(i64 %j) {
+entry:
+  %sum = add i64 %j, 1
+  ret i64 %sum
+}
+
+define i64 @k(i64 %k) {
+entry:
+  %sum = add i64 %k, 1
+  ret i64 %sum
+}
+
+define i64 @l(i64 %l) {
+entry:
+  %sum = add i64 %l, 1
+  ret i64 %sum
+}
+
+define i1 @loadb() {
+entry:
+  %b = load i1* @b
+  ret i1 %b
+}
+
+define i1 @loada() {
+entry:
+  %a = load i1* getelementptr ([4 x i1]* @a, i64 0, i64 2)
+  ret i1 %a
+}
+
+define i32 @f32(i32 %a, i32 %b, i32 %c, i32 %d) {
+entry:
+  br label %first
+
+second:
+  %eh = mul i32 %e, %h
+  %sum = add i32 %eh, %ef
+  br label %exit
+
+exit:
+  %product = phi i32 [%ef, %first], [%sum, %second]
+  ret i32 %product
+
+first:
+  %e = add i32 %a, 7
+  %f = add i32 %b, 7
+  %g = add i32 %c, 8
+  %h = add i32 %d, 8
+  %ef = mul i32 %e, %f
+  %gh = mul i32 %g, %h
+  %gotosecond = icmp slt i32 %gh, -9
+  br i1 %gotosecond, label %second, label %exit
+}