AMDGPU/SI: Make sure to emit TargetConstant nodes when matching ds_*permute

Summary:
This fixes a bug with ds_*permute instructions where if it was passed a
constant address, then the offset operand would get assigned a register
operand instead of an immediate.

Reviewers: scchan, arsenm

Subscribers: arsenm, llvm-commits

Differential Revision: http://reviews.llvm.org/D19994

llvm-svn: 272349