PNaCl: Fix handling of va_arg on structs in simplification passes

Cherry picked from https://codereview.chromium.org/135953002, originally
authored by mseaborn@chromium.org

Without this ordering change, ExpandStructRegs will fail with:

Value:   %val = va_arg i8* %va_list, %MyStruct
LLVM ERROR: Unrecognized struct value

BUG=https://code.google.com/p/nativeclient/issues/detail?id=2381
TEST=*.ll tests
R=dschuff@chromium.org