[x86 fast-isel] Materialize allocas with the correct-sized lea for ILP32

Summary:
X86FastISel::fastMaterializeAlloca was incorrectly conditioning its
opcode selection on subtarget bitness rather than pointer size.

Differential Revision: http://reviews.llvm.org/D6136

git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@221386 91177308-0d34-0410-b5e6-96231b3b80d8