[RS4GC] Update and simplify handling of Constants in findBaseDefiningValueOfVector().

[mjacob]

Summary

This is analogous to r256079, which removed an overly strong assertion, and
r256812, which simplified the code by replacing three conditionals by one.

[reames]

LGTM w/a test case or two added.

[mjacob]

Add test case.

lib/Transforms/Scalar/RewriteStatepointsForGC.cpp

 findBaseDefiningValueOfVector(Value *I) {
   // Each case parallels findBaseDefiningValue below, see that code for
   // detailed motivation.
 
   if (isa<Argument>(I))
     // An incoming argument to the function is a base pointer
     return BaseDefiningValueResult(I, true);
 
-  // We shouldn't see the address of a global as a vector value?
-  assert(!isa<GlobalVariable>(I) &&
-         "unexpected global variable found in base of vector");
-
-  // inlining could possibly introduce phi node that contains
-  // undef if callee has multiple returns
-  if (isa<UndefValue>(I))
-    // utterly meaningless, but useful for dealing with partially optimized
-    // code.
+  if (isa<Constant>(I))
+    // Constant vectors consist only of constant pointers.
     return BaseDefiningValueResult(I, true);
 
-  // Due to inheritance, this must be _after_ the global variable and undef
-  // checks
-  if (Constant *Con = dyn_cast<Constant>(I)) {
-    assert(!isa<GlobalVariable>(I) && !isa<UndefValue>(I) &&
-           "order of checks wrong!");
-    assert(Con->isNullValue() && "null is the only case which makes sense");
-    return BaseDefiningValueResult(Con, true);
-  }
-  
   if (isa<LoadInst>(I))
     return BaseDefiningValueResult(I, true);
 
   if (isa<InsertElementInst>(I))
     // We don't know whether this vector contains entirely base pointers or
     // not.  To be conservatively correct, we treat it as a BDV and will
     // duplicate code as needed to construct a parallel vector of bases.
     return BaseDefiningValueResult(I, false);

test/Transforms/RewriteStatepointsForGC/constants.ll

 ; CHECK-LABEL: use:
 ; CHECK: gc.statepoint
 ; CHECK: gc.relocate
   call token (i64, i32, void ()*, i32, i32, ...) @llvm.experimental.gc.statepoint.p0f_isVoidf(i64 0, i32 0, void ()* @foo, i32 0, i32 0, i32 0, i32 0)
   %res = load i8 addrspace(1)*, i8 addrspace(1)* addrspace(1)* %addr2, align 1
   ret i8 addrspace(1)* %res
 }
 
-
+; Globals don't move and thus don't get relocated
+define i8 addrspace(1)* @test5(i1 %always_true) gc "statepoint-example" {
+; CHECK-LABEL: @test5
+; CHECK: gc.statepoint
+; CHECK-NEXT: %res = extractelement <2 x i8 addrspace(1)*> <i8 addrspace(1)* @G, i8 addrspace(1)* @G>, i32 0
+entry:
+  call token (i64, i32, void ()*, i32, i32, ...) @llvm.experimental.gc.statepoint.p0f_isVoidf(i64 0, i32 0, void ()* @foo, i32 0, i32 0, i32 0, i32 0)
+  %res = extractelement <2 x i8 addrspace(1)*> <i8 addrspace(1)* @G, i8 addrspace(1)* @G>, i32 0
+  ret i8 addrspace(1)* %res
+}