Fix a bug in our autoupgrade support: in an argument list to a function

call, we should treat "i64 zext" as the start of a constant expr, but
"i64 0 zext" as an argument with an obsolete attribute on it (this form
is already tested by test/Assembler/2007-07-30-AutoUpgradeZextSext.ll).

Make the autoupgrade logic more discerning to avoid treating "i64 zext"
as an old-style attribute, causing us to reject a valid constant expr.
This fixes PR3876.

llvm-svn: 67682