Fix for PR1633: Verifier doesn't fully verify GC intrinsics

LLVM now enforces the following prototypes for the write barriers:

<ty>* @llvm.gcread(<ty2>*, <ty>**)
void @llvm.gcwrite(<ty>*, <ty2>*, <ty>**)

And for @llvm.gcroot, the first stack slot is verified to be an alloca or a 
bitcast of an alloca.

Fixes test/CodeGen/Generic/GC/lower_gcroot.ll, which violated these.

llvm-svn: 42051