Improve codegen for the LLVM offsetof/sizeof "operator". Before we compiled

Improve codegen for the LLVM offsetof/sizeof "operator".  Before we compiled
this LLVM function:

int %foo() {
        ret int cast (int** getelementptr (int** null, int 1) to int)
}

into:

foo:
        mov %EAX, 0
        lea %EAX, DWORD PTR [%EAX + 4]
        ret

now we compile it into:

foo:
        mov %EAX, 4
        ret

This sequence is frequently generated by the MSIL front-end, and soon the malloc lowering pass and
Java front-ends as well..

-Chris

git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@14834 91177308-0d34-0410-b5e6-96231b3b80d8