[mips] Sign-extend i32 values truncated from previously zero-extended i32 values.

Summary:
This is a special case for MIPS64 because the architecture requires
properly 32-bit sign-extended values in the register containers.

Additionaly, we merge consecutive trunc + AssertZExt nodes in order
to avoid unnecessary sign-extensions when the extension comes from a
type smaller than i32.

Reviewers: dsanders

Subscribers: dsanders, sdardis, llvm-commits

Differential Revision: http://reviews.llvm.org/D18893

llvm-svn: 266203