Use less byval on 32-bit Windows x86 for classes with bases

This comes up in V8, which has a Handle template class that wraps a
typed pointer, and is frequently passed by value. The pointer is stored
in the base, HandleBase. This change allows us to pass the struct as a
pointer instead of using byval. This avoids creating tons of temporary
allocas that we copy from during call lowering.

Eventually, it would be good to use FCAs here instead.

llvm-svn: 291917