AMDGPU/SI: Use InstAlias instead of MnemonicAlias for VOPC instructions

Summary:
With InstAlias, we don't need to print the _e32 portion of the mnemonic
when we print the $dst operand.  This change makes it possible to
include vcc in the asm string when we switch VOPC over to having
implicit vcc defs.

Reviewers: arsenm

Subscribers: llvm-commits

Differential Revision: http://reviews.llvm.org/D11813

llvm-svn: 244362