[dfsan] Add explicit zero extensions for shadow parameters in function wrappers.

In the case where dfsan provides a custom wrapper for a function,
shadow parameters are added for each parameter of the function.
These parameters are i16s. For targets which do not consider this
a legal type, the lack of sign extension information would cause
LLVM to generate anyexts around their usage with phi variables
and calling convention logic.

Address this by introducing zero exts for each shadow parameter.

Reviewers: pcc, slthakur

Differential Revision: https://reviews.llvm.org/D33349

llvm-svn: 311087