Add @llvm.assume, lowering, and some basic properties

This is the first commit in a series that add an @llvm.assume intrinsic which
can be used to provide the optimizer with a condition it may assume to be true
(when the control flow would hit the intrinsic call). Some basic properties are added here:

 - llvm.invariant(true) is dead.
 - llvm.invariant(false) is unreachable (this directly corresponds to the
   documented behavior of MSVC's __assume(0)), so is llvm.invariant(undef).

The intrinsic is tagged as writing arbitrarily, in order to maintain control
dependencies. BasicAA has been updated, however, to return NoModRef for any
particular location-based query so that we don't unnecessarily block code
motion.

git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@213973 91177308-0d34-0410-b5e6-96231b3b80d8

test/Transforms/InstSimplify/assume.ll

@@ -0,0 +1,13 @@
+; RUN: opt -instsimplify -S < %s | FileCheck %s
+
+define void @test1() {
+  call void @llvm.assume(i1 1)
+  ret void
+
+; CHECK-LABEL: @test1
+; CHECK-NOT: llvm.assume
+; CHECK: ret void
+}
+
+declare void @llvm.assume(i1) nounwind
+