Add 'musttail' marker to call instructions

This is similar to the 'tail' marker, except that it guarantees that
tail call optimization will occur.  It also comes with convervative IR
verification rules that ensure that tail call optimization is possible.

Reviewers: nicholas

Differential Revision: http://llvm-reviews.chandlerc.com/D3240

git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@207143 91177308-0d34-0410-b5e6-96231b3b80d8

test/Bitcode/tailcall.ll

@@ -0,0 +1,17 @@
+; RUN: llvm-as < %s | llvm-dis | FileCheck %s
+
+; Check that musttail and tail roundtrip.
+
+declare cc8191 void @t1_callee()
+define cc8191 void @t1() {
+; CHECK: tail call cc8191 void @t1_callee()
+  tail call cc8191 void @t1_callee()
+  ret void
+}
+
+declare cc8191 void @t2_callee()
+define cc8191 void @t2() {
+; CHECK: musttail call cc8191 void @t2_callee()
+  musttail call cc8191 void @t2_callee()
+  ret void
+}