AMDGPU/SI: Add 64-bit versions of v_nop and v_clrexcp

Summary:
The assembly printing of these is still missing the encoding size
suffix, but this will be fixed in a later commit.

Reviewers: arsenm

Subscribers: arsenm, llvm-commits

Differential Revision: http://reviews.llvm.org/D13436

llvm-svn: 249424