AMDGPU/SI: Add a helper for creating aliases for the _e32 instructions

Summary:
We are currently only using these aliases for VOPC instructions,
but this helper will make it easier to use them everywhere.

These aliases allow for the automatic matching of instructions
with forced 32-bit encoding.  Eventually, we should be able to remove
the custom C++ logic we have for this in the assembler.

Reviewers: arsenm

Subscribers: arsenm, llvm-commits

Differential Revision: http://reviews.llvm.org/D13396

llvm-svn: 249330